Query Tips

Q. Execute an Operating System Command From Within SQL Server (Completed 36) A. The xp_cmdshell extended store procedure makes it possible to execute operating system commands from within SQL Server. Example: EXEC Master..xp_cmdshell 'Dir c:\' Q. How can I create a plain-text flat file from SQL Server as input to another application? A. One of the purposes of Extensible Markup Language (XML) is to solve challenges like this, but until all applications become XML-enabled, consider using our faithful standby, the bulk copy program (bcp) utility. This utility can do more than just dump a table; bcp also can take its input from a view instead of from a table. After you specify a view as the input source, you can limit the output to a subset of columns or to a subset of rows by selecting appropriate filtering (WHERE and HAVING) clauses. More important, by using a view, you can export data from multiple joined tables. The only thing you cannot do is specify the sequence in which the rows are written to the flat file, because a view does not let you include an ORDER BY clause in it unless you also use the TOP keyword. If you want to generate the data in a particular sequence or if you cannot predict the content of the data you want to export, be aware that in addition to a view, bcp also supports using an actual query. The only "gotcha" about using a query instead of a table or view is that you must specify queryout in place of out in the bcp command line. For example, you can use bcp to generate from the pubs database a list of authors who reside in California by writing the following code: bcp "SELECT * FROM pubs..authors WHERE state = 'CA'" queryout c:\CAauthors.txt -c -T -S Q. How can I programmatically detect whether a given connection is blocked? A. A connection is blocked when it requires an object that another connection has a lock on. You can use the system stored procedure sp_lock to retrieve information about the current locks in SQL Server, and you can use the server process ID (SPID) to filter the information that sp_lock returns. To determine whether a given process is waiting for the release of a locked resource, you can execute the sp_GetBlockInfo procedure that follows. Note: You must execute the procedure before the timeout. USE master GO CREATE PROCEDURE sp_GetBlockInfo @BlockedSPID as int AS IF EXISTS (select * FROM master.dbo.syslockinfo WHERE req_spid = @BlockedSPID AND req_status = 3) SELECT sli1.req_spid AS SPID, SUBSTRING (u.name, 1, 8) As Mode, DB_NAME(sli1.rsc_dbid) AS [Database], OBJECT_NAME(sli1.rsc_objid) AS [Table], sli1.rsc_Text AS [Resource] FROM master.dbo.syslockinfo sli1 JOIN master.dbo.spt_values u ON sli1.req_mode + 1 = u.number AND u.type = 'L' JOIN master.dbo.syslockinfo sli2

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ON sli1.rsc_dbid = sli2.rsc_dbid AND sli1.rsc_objid = sli2.rsc_objid AND sli1.rsc_text = sli2.rsc_text WHERE sli2.req_spid = @BlockedSPID AND sli1.req_status = 1 AND sli1.req_spid <> @BlockedSPID AND sli2.req_status = 3 ELSE SELECT CAST(1 as int) AS SPID, SUBSTRING ('', 1, 8) AS Mode, DB_NAME(NULL) AS [Database], OBJECT_NAME(NULL) AS [Table], CAST(NULL AS nchar(32)) AS [Resource] WHERE 1=2 GO The sp_GetBlockInfo procedure tells you the lock mode, the database and object names of the locked resource, and in the case of a blocking chain, which SPID is the root blocker. If the process is not blocked, sp_GetBlockInfo returns an empty recordset. You can also detect blocks by checking for error 1222, "Lock request time out period exceeded." The LOCK_TIMEOUT setting controls how long a process will wait for locks to be released before timing out. When the lock timeout occurs, SQL Server sends error 1222 to the application. In SQL Server 7.0, this error aborts the statement but does not cause the batch to roll back, so you can look for the Transact-SQL system variable @@ERROR and determine where locks exist Q. Can you create UNIQUE and PRIMARY KEY constraints on computed columns in SQL Server 2000? A. In SQL Server, the physical mechanism that UNIQUE and PRIMARY KEY constraints use to enforce uniqueness is a unique index. Because SQL Server 2000 supports indexes on computed columns, you can create UNIQUE and PRIMARY KEY constraints on computed columns. Defining a UNIQUE constraint on a computed column is a straightforward process, as the following example shows: CREATE TABLE T1 ( col1 int NOT NULL, col2 AS col1 + 1 UNIQUE ) However, if you define a PRIMARY KEY on a computed column, such as: CREATE TABLE T2 ( col1 int NOT NULL, col2 AS col1 + 1 PRIMARY KEY ) you receive the following error: Server: Msg 8111, Level 16, State 2, Line 1 Cannot define PRIMARY KEY constraint on nullable column in table 'T2'. Server: Msg 1750, Level 16, State 1, Line 1 Could not create constraint. See previous errors. Because of the primary key constraint, SQL Server requires you to guarantee that your computation's result will not be NULL. The computation in the computed column can overflow (for example, when you add 1 to the largest integer) or underflow (when you subtract 1 from the smallest integer), and other computations can result in a divide-by-zero error. However, if the ARITHABORT (which determines whether a query has ended when an overflow or a divide-by-zero error occurs)

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and ANSI_WARNINGS (which specifies ANSI SQL-92 standard behavior for several error conditions) session settings are off, instead of ending the query, the computation can have a NULL result. In practice, when either ARITHABORT or ANSI_WARNINGS settings is off, you cannot create an index on a computed column or insert values into a table that has an index on a computed column because SQL Server detects such an attempt and returns an error. But SQL Server still requires you to guarantee that the computation will not result in NULL values. The trick is to wrap the computed column's computation with the ISNULL() function and supply an alternative value if the computation results in NULL: CREATE TABLE T2 ( col1 int NOT NULL, col2 AS ISNULL(col1 + 1, 0) PRIMARY KEY ) Q. Why does my inline or embedded SQL run faster than my stored procedures? A. Recompilations might be the source of the slower stored procedure speed. To find out for sure, you need to do some performance investigation, such as looking at Showplans for each type of query versus calling the stored procedures and comparing query plan cache hits to cache misses. You can also try coding the object owner for referenced tables, views, and procedures inside your stored procedures, as the following example shows: SELECT * FROM dbo.mytable This technique helps you reuse plans and prevent cache misses. Q. Why doesn't SQL Server permit an ORDER BY clause in the definition of a view? A. SQL Server excludes an ORDER BY clause from a view to comply with the ANSI SQL-92 standard. Because analyzing the rationale for this standard requires a discussion of the underlying structure of the structured query language (SQL) and the mathematics upon which it is based, we can't fully explain the restriction here. However, if you need to be able to specify an ORDER BY clause in a view, consider using the following workaround: USE pubs GO CREATE VIEW AuthorsByName AS SELECT TOP 100 PERCENT * FROM authors ORDER BY au_lname, au_fname GO The TOP construct, which Microsoft introduced in SQL Server 7.0, is most useful when you combine it with the ORDER BY clause. The only time that SQL Server supports an ORDER BY clause in a view is when it is used in conjunction with the TOP keyword. Note that the TOP keyword is a SQL Server extension to the ANSI SQL-92 standard. Q. Is using the TOP N clause faster than using SET ROWCOUNT N to return a specific number of rows from a query? A. With proper indexes, the TOP N clause and SET ROWCOUNT N statement are equally fast, but with unsorted input from a heap, TOP N is faster. With unsorted input, the TOP N operator uses a small internal sorted temporary table in which it replaces only the last row. If the input is nearly sorted, the TOP N engine must delete or insert the last row only a few times. Nearly sorted means you're dealing with a heap with ordered inserts for the initial population and without many updates, deletes, forwarding pointers, and so on afterward. A nearly sorted heap is more efficient to sort than sorting a huge table. In a test that used TOP N to sort a table with the same number of rows but with unordered inserts, TOP N was not as efficient anymore. Usually, the I/O time is the same both with an index and without; however, without an index SQL

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Server must do a complete table scan. Processor time and elapsed time show the efficiency of the nearly sorted heap. The I/O time is the same because SQL Server must read all the rows either way. Q, The Difference between 'Count' and 'Count(*)' A. 'Count': Counts the number of non-null values. 'Count(*)': Counts the number of rows in the table, including null values and duplicates. Q. I have two tables t1 and t2 both with the columns a1, a2. I want to find the difference of (the set of t1) - (the set of t2) without using the keyword EXCEPT because MSSQL 2000 does not recognize that word. I have tried this query but it does not give me what I want: SELECT * FROM t1 WHERE NOT EXISTS (SELECT t1.* FROM t1 INNER JOIN t2 ON t1.a1=t2.a1 AND t1.a2=t2.a2) A.SELECT * FROM t1 WHERE NOT EXISTS (SELECT * FROM t2 ON t1.a1=t2.a1 AND t1.a2=t2.a2) This is the solution. The difference of (the set of t1) - (the set of t2) is SELECT * FROM t1 LEFT OUTER JOIN t2 ON t1.a1=t2.a1 AND t1.a2=t2.a2 WHERE t2.a1 IS NULL AND t2.a2 IS NULL According to SQL Query Analyzer, this is slightly more efficient than the left join (possibly only because of the tables I tested it with): SELECT * FROM t1 WHERE NOT EXISTS (SELECT * FROM t2 WHERE t1.a1=t2.a1 AND t1.a2=t2.a2) Q, Leading Zero's in Stored Procedure I have a form and I am passing a value to a stored procedure. The value has leading zeros. When the values are passed the leading zeros are dropped, thus causing my stored procedure to blow up. Is there a way to maintain those zeros in passing or pick them up again in the procedure? Try to pass value as string. If you were passing a string value then the LZ should not be dropped, so I suspect that you are passing a numeric value then converting it back to a varchar which will drop any LZ. If you don't want to change the interface then you can always restore the LZ in the stored procedure by using the following. declare @lz varchar(10) declare @numeric numeric(10,2) declare @result varchar(20) select @lz = '0000000000' select @numeric = 0000123.45 -- the LZ will be lost upon assignment select @result = substring(@lz, 1, datalength(@lz)-datalength(convert(varchar Q, Method to Perform Case Sensitive Searches in SQL Server A, By default, SQL Server 7.0 installation sets up SQL Server to run in a case insensitive mode. For most applications this may work great, but there are certain situations were case sensitive searches are required. For instance, if a web site needs to have passwords that are case sensitive a method needs to be devised to perform case-sensitive queries. The following script performs a case-sensitive search. First let’s create a table: CREATE TABLE test ( id INT NOT NULL, msg VARCHAR(100) NOT NULL )

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Now let’s insert some case sensitive data into it: INSERT INTO test (id, msg) VALUES (1, 'bUSY'); INSERT INTO test (id, msg) VALUES (2, 'BUSY'); INSERT INTO test (id, msg) VALUES (3, 'busy'); In our test we are searching for a ‘bUSY’ value in the msg column of the test table. So the syntax of the same query, if the SQL Server was set to be case sensitive, would be: SELECT * FROM test where msg = ‘bUSY’; This query will return all rows in the test table. Now, here is the script that will perform the case sensitive search. DECLARE @table VARCHAR( 30 ) DECLARE @col VARCHAR( 30 ) DECLARE @searchVal VARCHAR( 195 ) SET @table = 'test' SET @col = 'msg' SET @searchVal = 'bUSY' DECLARE @sql VARCHAR( 255 ) DECLARE @colLength VARCHAR( 3 ) SELECT @colLength = CONVERT( varchar(3), DATALENGTH( @searchVal ) ) SELECT @sql = 'SELECT * ' + ' FROM ' + @table + ' WHERE' + ' CONVERT( varbinary( ' + @colLength + '), ' + @col + ') = ' + ' CONVERT( varbinary( ' + @colLength + '), "' + @searchVal + '")' + ' AND ' + @col + '="' + @searchVal + '"' EXEC( @sql ) Listing Available Tables Q, How do I list the available tables in a database I'm querying? A, You can get a list of tables in a database by calling the INFORMATION_SCHEMA.Tables view like this: SELECT * FROM information_schema.tables This returns the name of the current database, the owner, the table name, and the table type for each table in the database. It's possible to query the system tables directly, but if this gives the information you need, it's better to use the existing views that come with SQL Server. Question: How do I count the number of duplicate items in a table? Answer: Lets break down your question into several steps. First, let's create a sample table using the following code: create table dups ( i int ) go declare @i int

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@i = 0 while (@i < 35) begin insert into dups(i) values (cast (rand() * 50 as int)) select @i = @i + 1 end Now, let's find rows that are duplicates. For that we can use a simple group by statement: select i,count(*) as num_records from dups group by i having count(*) > 1 My sample data produced the following result set: i num_records ----------- ----------0 2 5 2 18 2 22 2 27 2 31 2 34 2 44 2 49 2 This identifies the rows that have duplicates. But it does not return the total number of duplicates in the table. The first change we must make is to recognize the above rows that show 2 contain only one duplicate. So we want a query that basically sums up the duplicates from the above query. To do so, we take the previous query and can put that in the from statement as a derived table. We then can use the sum function to create the total for us: select sum(num_dups) from (select i,count(*)-1 as num_dups from dups group by i having count(*)-1 > 0) as mydups Using Distributed Queries on SQL Server 7.0 SQL Sever 7 allows developers to execute commands against OLE DB data sources on different servers. In order to execute commands on remote servers, the SQL Server instance where the commands will be issued must be setup properly. This entails adding the remote server to SQL Server's linked server list. Do this by using the sp_addlinkedserver command. For example, to link a remote SQL Server database that resides on the RemoteDBServer server, you would usethe following syntax: EXEC master. . sp_addlinkedserver @server = 'RemoteDBServer', @provider = 'SQLOLEDB', @srvproduct = 'SQL Server', @datasrc = 'RemoteDBServer' Note that only members of the sysadmin role can set this server option. Once the remote database has been linked, queries can be executed against it as long as the remote tables are prefaced using Server.Database.Table Owner.Table Name. For example, the following query would return all rows in the authors table of our RemoteDBServer SQL Server database:

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SELECT * FROM RemoteDBServer.pubs.dbo.authors Dynamic Query with .... in (@var) Question: Can I use a variable in a query with the IN clause (a,b,c..z), without getting quotes or conversion errors? Answer: You can use a variable as your IN clause, but this requires that you use the EXEC function to run the statement. Delete Files from SQL Server Local Machine Ever wanted to delete files from the local machine that your SQL Server database is running? You can do it using the extended stored procedure xp_cmdshell like this: EXEC master..xp_cmdshell 'del C:\file.txt' But this requires the sysadmin option on the SQL Server and Admin role from NT server. In most instances it is not preferable to give these privileges. So to delete files without requiring this access use the built-in SQL Server Automation APIs and the FileSystemObject: DECLARE DECLARE

@hr @ole_FileSystem

int int

EXEC @hr = sp_OACreate 'Scripting.FileSystemObject', @ole_FileSystem OUT EXEC @hr = sp_OAMethod @ole_FileSystem, 'DeleteFile', NULL, 'C:\file.txt' EXEC @hr = sp_OADestroy @ole_FileSystem Large Text Fields Question: How do I create a text field of greater than 8,000 characters (in v7.0)? I attempted to use the "text" data type, but my code returned an error saying the maximum size was 8,000. Here's the code: CREATE TABLE X ( X_ID int IDENTITY(1,1), X_DESC text (60000) NOT NULL ) GO Answer: SQL Server is returning a bogus error message. The real error has to do with your syntax. When specifying text you don't specify a size. You can see the real error message if you reduce the number 60000 to 5. Then you will get this message: Server: Msg 2716, Level 16, State 1, Line 1 Column or parameter #2: Cannot specify a column width on data type text. Instead, simply specify it as text without the parentheses and the number. The actual size of the storage used for the text field will depend on how much data you actually put in the column Importing Excel into SQL without Using Wizard Question: How do I give a user the option of importing Excel and a delimited text file into a SQL Server Database without manually using SQL DTS? Answer: You can use the DTS object model to programmatically create, modify and run DTS packages. You can do this many ways, but essentially this object model has objects for anything you can do in DTS. If you already have the package created and saved as a file, add a reference to the DTS Library. Then you can call it like this: Dim Package As New DTS.Package Package.LoadFromStorageFile App.Path "\DTSPackage.dts" Package.Execute

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Connecting to SQL Server 7 via MS-DOS 6.2 Question: I have some complex engine control software that has to run under MS-DOS 6.2, but it needs to transfer data to SQL Server 7. What's the best way to connect to SQL Server from DOS? Answer: You can use OSQL or ISQL to connect to SQL Server from the command line. Both of these utilities are Win32 command-line utilities that allow you to log in to a server and issue any SQL command. The OSQL utility supports the most functionality and uses ODBC to connect to SQL Server. The syntax "osql/?" provides a listing of all OSQL flags. This syntax connects to a server and issues a query: osql -S "servername" -U "sa" -d "pubs" -q "SELECT * FROM Authors" To insert data into a database, you could put OSQL commands in a batch file or provide an input file of SQL INSERT commands. See the SQL Server BOL for details and syntax of OSQL. If you have a straight DOS box, you need to install the 16-bit client that comes with SQL Server 6.5. Both the client and the 6.5 version of ISQL use DB-Lib, which doesn't support some SQL Server 7 features like unicode columns, varchar columns larger than 255, etc. Passing a CSV List Within an IN Statement Question: I have a CSV list I am passing through as a parameter in a stored procedure. Example of Region list: 1,2,3,4,5,6,7,8,9,10 CREATE PROCEDURE sp_getReport @Regionlist varchar(100) AS Select * from tblRegion where Region_ID IN (@regionlist) The varchar treats the list as one block of data. How can I change my code/syntax to read each value of the list separately within the IN statement? Answer: When you pass variables that are part of the statement in a stored procedure, you need to use the EXEC(UTE) statement. Here's the syntax to call the stored procedure and modify the current stored procedure to take the regions list as a parameter (the only difference is I called the column RegionID instead of Region_ID): sp_getreport '1,2,3,4,5,6,7,8,9,10' CREATE PROCEDURE sp_getReport @regionlist varchar(100) AS EXEC ('Select * from tblRegion where RegionID IN (' + @regionlist +')') Creating Tables with Variable Names Question: Is it possible to create tables in stored procedures using a variable for the table name? For instance: declare @MyName set @MyName = 'dbo.Unique_Name' create table @MyName .... This obviously doesn't work, but does this ability exist? Answer: Yes, you can issue DLL statements with variables; you just need to use the EXECUTE statement. For example, this statement uses a variable for the table name, as in your example: DECLARE @TableName varchar(255) SET @TableName = '[dbo].[TestTable]'

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EXEC ('CREATE TABLE ' + @TableName + ' ( [ID] [int] NULL , [ItemDesc] [char] (50) NULL )') Use Sysobjects in SQL Server to Find Useful Database Information SQL Server sysobjects Table contains one row for each object created within a database. In other words, it has a row for every constraint, default, log, rule, stored procedure, and so on in the database. Therefore, this table can be used to retrieve information about the database. We can use xtype column in sysobjects table to get useful database information. This column specifies the type for the row entry in sysobjects. For example, you can find all the user tables in a database by using this query: select * from sysobjects where xtype='U' Similarly, you can find all the stored procedures in a database by using this query: select * from sysobjects where xtype='P' This is the list of all possible values for this column (xtype): C = CHECK constraint D = Default or DEFAULT constraint F = FOREIGN KEY constraint L = Log P = Stored procedure PK = PRIMARY KEY constraint (type is K) RF = Replication filter stored procedure S = System table TR = Trigger U = User table UQ = UNIQUE constraint (type is K) V = View X = Extended stored procedure Query to a Text File Question: I would like to create a stored procedure that runs a query and outputs the results to a text file and allows me to add extra delimeters and static field info. Answer: SQL Server has no native command for outputting query results to a file. You can use the extended stored procedure xp_cmdshell and call isql (command-line SQL) with your query and output the results to a file. Any delimiters would need to be part of the SELECT string: DECLARE @isqlString varchar(255) SELECT @isqlString = 'isql -Q "SELECT DateCol FROM NorthwindTest.dbo.Test" -E -o C:\Results.txt' EXEC master..xp_cmdshell @isqlString Dynamic Variables and Stored Procedures Question: I need to pass the @TheVar variable to the stored procedure's IN statement but I keep getting zero records. If I type this statement into the SQL... WHERE Name IN ('John', 'Frank', 'Tom') the statement works fine; 20 records are returned. EXECUTE the_proc "'John', 'Frank', 'Tom'"

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CREATE PROCEDURE the_proc @TheVar nvarchar(40) AS SELECT COUNT(FieldName) FROM Clients WHERE Name IN (@TheVar) Do you have any ideas? I use MS SQL Server 7.0 SP2. Answer: If you want to use variables as part of your SQL statement you need to modify it to call the EXECUTE statement. I created a Clients table with a single Name field of varchar(255) with records for John, Frank, and Tom. Then I created a stored procedure (similar to yours) that uses the input variable to build the IN clause like this: CREATE PROCEDURE GetClients @TheVar varchar(255) AS EXEC ('SELECT COUNT(Name) FROM Clients WHERE Name IN (' + @TheVar + ')') As you can see, it builds the SELECT statement using the variable and then the EXEC statement runs it. The syntax to call this procedure is: GetClients "'John', 'Tom', 'Frank'" --3 Question: If my SQL Server has the following specs: 6.05.02 SQL-DMO 6.50.252 DB-Library Which version do I have? And which service pack version do I have? Answer: If you want to query the version of SQL Server that's currently running you can use the @@version variable: SELECT @@version This returns the version, processor, build and service pack information for the currently installed SQL Server. This information is stored in the system tables, and you can retrieve more details by calling the extended stored procedure xp_msver. Be sure to call it from the master database. Data Type Question: How do I compare two values when one value(data type) is char and the other is an integer? Answer: For this you can use the CONVERT function. Since you can't convert character data to numbers, you'll have to convert the integer column to character. Assuming you have a table called TestTable with two columns: IntCol int CharCol char(10) with these values IntCol CharCol ------ ------1 1 2 2 3 xyz 4 5 You can use this query to SELECT rows based on a comparison: SELECT * FROM TestTable WHERE CONVERT(char(10), IntCol) = CharCol IntCol

CharCol

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----------- ---------1 1 2 2 E-mail in a Query Question: How do I send an e-mail to myself if a field reads "SEND_MAIL?" For example, a user wants more info on a product, so he clicks on the Send Mail button, which in turn throws an entry into the database. This entry is SEND_MAIL. Now at the end of the day I would like the server to scan the database for the word SEND_MAIL and, when it finds one, send me an email with the name and email address of the person. Answer: To answer this question I took the scenario of a Web site that logs user registrations and puts a visitor's first name, last name and e-mail address in a table. I then created a stored procedure that selects this data and e-mails it to an e-mail address provided as an input parameter. If the SendMail column in the table is Y, then the details will be e-mailed. The stored procedure is called like this: usp_CheckForMail '[email protected]' The script to create the table is as follows: if exists (select * from sysobjects where id = object_id(N'[dbo].[TestTable]') and OBJECTPROPERTY(id, N'IsUserTable') = 1) drop table [dbo].[TestTable] GO CREATE TABLE [dbo].[TestTable] ( [ID] [int] IDENTITY (1, 1) NOT NULL , [Fname] [varchar] (50) NULL , [Lname] [varchar] (50) NULL , [EmailAddress] [varchar] (100) NULL , [SendMail] [char] (1) NULL ) ON [PRIMARY] GO The script to create the stored procedure to send the e-mails is: if exists (select * from sysobjects where id = object_id(N'[dbo].[usp_CheckForMail]') and OBJECTPROPERTY(id, N'IsProcedure') = 1) drop procedure [dbo].[usp_CheckForMail] GO SET QUOTED_IDENTIFIER ON SET ANSI_NULLS ON GO CREATE PROCEDURE usp_CheckForMail @SendToEmail varchar(255) AS DECLARE @ID int, @Fname varchar(50), @Lname varchar(50), @EmailAddress varchar(100), @MsgText varchar(255) DECLARE WebVisitors CURSOR FOR SELECT ID, Lname, Fname, EmailAddress FROM TestTable WHERE SendMail = 'Y' ORDER BY Lname, Fname OPEN WebVisitors FETCH NEXT FROM WebVisitors INTO @ID, @Lname, @Fname, @EmailAddress

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EXEC master.dbo.xp_startmail /* typically mail will be started on your server */ -- Check @@FETCH_STATUS to see if there are any more rows to fetch. WHILE @@FETCH_STATUS = 0 BEGIN -- PRINT 'Visitor: ' + @Fname + ' ' + @Lname + ' at ' + @EmailAddress + ' [ID=' + CAST(@ID AS varchar) + ']' SET @MsgText = 'Visitor: ' + @Fname + ' ' + @Lname + ' at ' + @EmailAddress + ' [ID=' + CAST(@ID AS varchar) + ']' EXEC master.dbo.xp_sendmail @recipients = @SendToEmail, @message =@MsgText, @subject = 'New Web Visitor' FETCH NEXT FROM WebVisitors INTO @ID, @Lname, @Fname, @EmailAddress END CLOSE WebVisitors DEALLOCATE WebVisitors GO SET QUOTED_IDENTIFIER OFF SET ANSI_NULLS ON GO Pros and Cons of Extended Stored Procedures Question: What are the pros and cons of creating extended stored procedures? Answer: The pros are that you can implement additional functionality and access data from DLLs from within SQL Server. If you need to do something that can be done only in C or C++, or if you have data that can be accessed only outside of SQL Server, you can still provide a link to it. The biggest con to extended stored procedures is that they run in the same process space as SQL Server. So an errant DLL could overwrite memory and cause SQL Server to crash or even corrupt data. The biggest safeguard against these problems is thorough testing of the procedure Percent Sign in SQL Question: I have a table in which the key field has a value stored with a percent sign, like '1234%'. Using this value, I want to select from another table that can have values like '1234567', '1234678' and '1234098'. How do I go about it? Answer: The percent sign (%) is a wildcard in SQL Server. It can be used at the beginning or end of a string. So the following syntax will return all of the records you mentioned: SELECT * FROM TestTable WHERE Col LIKE '1234%' Col ------1234567 1234678 1234098 If you want to do an exact match for '1234' without the percent sign, then you'll have to trim off the last character, like this: SELECT * FROM TestTable WHERE Col LIKE LEFT('1234%', (LEN('1234%')-1)) Delete Duplicate Rows with a Single SQL Statement Question: Is it possible to delete duplicate rows in a table without using a temporary table (i.e., just do it with a single SQL statement)?

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Answer: All you need to do is compare the table to itself to find out which candidates are duplicates. Do this by assigning aliases to the table so you can use it twice, once as A and again as B, like this: delete from jobs where job_desc in ( select a.job_desc from jobs a, jobs b where a.job_desc = b.job_desc group by a.job_desc having count(a.job_desc) >1 ) When you do this you'll get a count based on the column value you think is duplicated. I used "desc" because the IDs will be different, so the description is the thing that is the candidate for repetition. Join the table to itself on that candidate to find matches of it. Everything will match to itself at least once that's why you group by the thing you think is a duplicate. Applying the HAVING clause to it squeezes out all the "ones" or singletons, leaving only the rows that have counts that are more than one in other words, your duplicate rows. By the way, this code trashes all the records that are duplicates. If you want to save one, add a comparison for the IDs to be different in the WHERE clause. Joining Queries from Oracle and SQL Server Databases Question: I need a query that retrieves info from an Oracle table and a query that retrieves info from a SQL Server table. The info has to be joined together according to Record ID numbers. I have very limited access to the Oracle database but full control of the SQL Server database.How do I join two different queries from two different databases? Answer: To query to different data sources, you can make the Oracle server a linked server to the SQL Server server. A linked server can be any OLE DB data source and SQL Server currently supports the OLE DB data provider for Oracle. You can add a linked server by calling sp_AddLinkedServer and query information about linked servers with sp_LinkedServers. An easier way to add a linked server is to use Enterprise Manager. Add the server through the Linked Servers icon in the Security node. Once a server is linked, you can query it using a distributed query (you have to specify the full name). Here's an example of a distributed query (from the SQL Server Books Online) that queries the Employees table in SQL Server and the Orders table from Oracle: SELECT emp.EmloyeeID, ord.OrderID, ord.Discount FROM SQLServer1.Northwind.dbo.Employees AS emp, OracleSvr.Catalog1.SchemaX.Orders AS ord WHERE ord.EmployeeID = emp.EmployeeID AND ord.Discount > 0 Use the OpenRowSet Function to Run a Query on a Remote SQL Server You can use the OPENROWSET( ) function to run a query on a remote SQL server by using the following syntax:

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SELECT *FROM OPENROWSET('SQLOLEDB', 'remote_server_name'; 'sa'; 'password','SQL statement') Here replace 'remote_server_name' with the name of the remote server on which you want to run the query. If necessary, replace 'sa' and 'password' with the name and password of a SQL login ID you want to use to log in to the remote server. Finally, replace 'SQL statement' with the SQL statement you want to run on the remote server Check Whether a Global Temporary Exists in a SQL Database Checking whether a table exists in a Microsoft SQL Server database is easy. You can use this query: SELECT 'x' FROM sysobjects WHERE type = 'U' and NAME = 'mytable' But this query will not work while searching for global temporary tables. Global temporary tables are stored in tempdb. Use this syntax for the search: DECLARE @temp_table VARCHAR(100) SET @temp_table = '##my_temp_table' IF NOT EXISTS (SELECT 'x' FROM tempdb..sysobjects WHERE type = 'U' and NAME = @temp_table) PRINT 'temp table ' + @temp_table + ' does not exist' ELSE PRINT 'temp table ' + @temp_table + ' exists.' Note: You cannot search for local temporary tables (# prefix tables) in this way. This is because SQL Server appends a unique number to the name you supply. For example, if you specified "#temp," the name in sysobjects would be something like "#temp____1234." See Who Is Blocking Your SQL Server If you have ever monitored any blocking problems in SQL Server, you know that sp_who only shows you the spid (SQL Server's internal Process ID) that is causing the blocking for each spid that is blocked. Often a blocked spid is shown as causing blocking for another spid. To see the spid (or spids) that started the whole mess off, execute the following SQL: SELECT p.spid ,convert(char(12), d.name) db_name , program_name , convert(char(12), l.name) login_name , convert(char(12), hostname) hostname , cmd , p.status , p.blocked , login_time , last_batch , p.spid FROM master..sysprocesses p JOIN master..sysdatabases d ON p.dbid = d.dbid JOIN master..syslogins l ON p.suid = l.suid WHERE p.blocked = 0 AND EXISTS ( SELECT 1 FROM master..sysprocesses p2 WHERE p2.blocked = p.spid ) We built this into our own version of sp_who, called sp_hywho. See the listing below. Code for sp_hywho: if exists (select * from sysobjects where id = object_id('dbo.sp_hywho') and sysstat & 0xf = 4) drop procedure dbo.sp_hywho

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GO Create Procedure sp_hywho ( @vcDBName sysname = NULL ) AS SET NOCOUNT ON IF EXISTS ( SELECT 1 FROM master..sysprocesses p WHERE p.blocked = 0 AND EXISTS ( SELECT 1 FROM master..sysprocesses p2 WHERE p2.blocked = p.spid ) ) BEGIN PRINT "Blocking caused by:" PRINT "" SELECT p.spid ,convert(char(12), d.name) db_name , program_name , convert(char(12), l.name) login_name , convert(char(12), hostname) hostname , cmd , p.status , p.blocked , login_time , last_batch , p.spid FROM master..sysprocesses p JOIN master..sysdatabases d ON p.dbid = d.dbid JOIN master..syslogins l ON p.suid = l.suid WHERE p.blocked = 0 AND EXISTS ( SELECT 1 FROM master..sysprocesses p2 WHERE p2.blocked = p.spid ) AND (p.dbid = DB_ID( @vcDBName ) OR @vcDBName IS NULL) ORDER BY 2,IsNull(Ltrim(program_name),"ZZZZZZZZZ"),4,5 PRINT "" END SELECT p.spid ,convert(char(12), d.name) db_name , program_name , convert(char(12), l.name) login_name , convert(char(12), hostname) hostname , cmd , p.status , p.blocked , login_time , last_batch , p.spid FROM master..sysprocesses p JOIN master..sysdatabases d ON p.dbid = d.dbid JOIN master..syslogins l ON p.suid = l.suid WHERE (p.dbid = DB_ID( @vcDBName ) OR @vcDBName IS NULL) ORDER BY 2,IsNull(Ltrim(program_name),"ZZZZZZZZZ"),4,5 SET NOCOUNT OFF GO

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if exists (select * from sysobjects where id = object_id('dbo.sp_hywho') and sysstat & 0xf = 4) GRANT EXEC ON dbo.sp_hywho TO PUBLIC GO Getting the last identity value used Many times, you'll want to know the last identity (key) value that was used in an insert. The biggest reason for this is so that the same value can be reused when inserting a foreign key. This is done differently between SQL Server and DB2. In DB2, the identity values can be picked up by the application and reused using and Identity_Val_function(), which returns the most recently assigned value for an identity column. In SQL Server, the last identity value used in an insert can be retrieved with the @@identity function. Except Operator To find rows in one set that do not exist in another set, use the except operator (as defined in SQL-92 and SQL-99). For example, here's how you find column1 from Table1 that does not exist in column2 of Table2: Select column1 from Table1 Except Select column2 from Table2; The except operator will remove duplicates, and a single null value will be returned in the case of multiple null values. To return duplicates, use except all. Keep in mind, of course, that other proprietary implementations (such as Minus in Oracle) exist. Understanding the different SQL Server 7.0 versions To date, Microsoft has released six different versions of SQL Server 7.0. These versions include the Desktop, Standard, and Enterprise Editions, as well as the Developer, Microsoft Developer (MSDE), and Small Business Server Editions. While all of these versions are SQL Server 7.0, there are some key differences. First of all, you can run the Desktop Edition on Windows NT Workstation 4.0, Windows NT Server 4.0, and Windows 9x. The Desktop Edition doesn't support the Microsoft Search Service, OLAP Services, parallel queries, or transaction replication--and it can't be bought on its own. Instead, you must buy either the Standard or the Enterprise Edition to get the Desktop Edition. You can run the Standard Edition of SQL Server only on Windows NT Server 4.0 (or later). This version does support such features as the Microsoft Search Service, OLAP Services, parallel queries, and transactional replication. It also supports up to 4 CPUs and 2 GB of RAM. In contrast, the Enterprise Edition runs only on Windows NT Server 4.0 Enterprise Edition and supports Microsoft Cluster Server. It also supports all of the features supported in the Standard Edition--plus up to 32 CPUs and more than 2 GB of RAM. The Developer Edition of SQL Server is included with Visual Studio for developer use. This version supports a limited number of connections but does include debugging tools. The Microsoft Developer Edition (MSDE) of SQL Server is simply a run-time engine that's included as part of Microsoft Office 2000. Although the MSDE version includes some of the management utilities, it doesn't include all of them. The MSDE was designed for you to distribute as part of an application, not as a stand-alone product. Finally, the Small Business Server Edition is part of Microsoft's Small Business Server. This version of SQL Server is essentially the same as the Standard Edition but comes hard-coded with a limit of 100 users and a maximum database size of 10 GB. Display Amount of Disk Activity Generated by Transact-SQL Statements You can set SQL Server to display information regarding the amount of disk activity generated by T-SQL statements. This option displays the number of scans, the number of logical reads (pages accessed), and the number of physical reads (disk accesses) for each table referenced in the statement. This option also displays the number of pages written for each statement. When STATISTICS IO is ON, statistical information is displayed. When OFF, the information is not displayed. After this option is set ON, all subsequent T-SQL statements return the statistical information until the option is set to OFF. Here is the syntax: SET STATISTICS IO {ON | OFF}

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Derived Tables Use 'Derived tables' wherever possible, as they perform better. Consider the following query to find the second highest salary from the Employees table: SELECT MIN(Salary) FROM Employees WHERE EmpID IN ( SELECT TOP 2 EmpID FROM Employees ORDER BY Salary Desc ) The same query can be re-written using a derived table, as shown below, and it performs twice as fast as the above query: SELECT MIN(Salary) FROM ( SELECT TOP 2 Salary FROM Employees ORDER BY Salary DESC ) AS A This is just an example, and your results might differ in different scenarios depending on the database design, indexes, volume of data, etc. So, test all the possible ways a query could be written and go with the most efficient one. Benefits of Derived Tables The biggest benefit of using derived tables over using temporary tables is that they require fewer steps, and everything happens in memory instead of a combination of memory and disk. The fewer the steps involved, along with less I/O, the faster the performance. Here are the steps when you use a temporary table: 1) Lock tempdb database 2) CREATE the temporary table (write activity) 3) SELECT data & INSERT data (read & write activity) 4) SELECT data from temporary table and permanent table(s) (read activity) 5) DROP TABLE (write activity) 4) Release the locks Compare the above to the number of steps it takes for a derived table: 1) CREATE locks, unless isolation level of "read uncommitted" is used 2) SELECT data (read activity) 3) Release the locks Using derived tables instead of temporary tables reduces disk I/O and can boost performance. Now let's see how. Q, How to Run a Query on a Remote SQL Server A, Use the OpenRowSet Function to Run a Query on a Remote SQL Server You can use the OPENROWSET( ) function to run a query on a remote SQL server by using the following syntax: SELECT * FROM OPENROWSET('SQLOLEDB', 'remote_server_name'; 'sa'; 'password','SQL statement') Here replace 'remote_server_name' with the name of the remote server on which you want to run the query. If necessary, replace 'sa' and 'password' with the name and password of a SQL login ID you want to use to

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log in to the remote server. Finally, replace 'SQL statement' with the SQL statement you want to run on the remote server Q, Why does my inline or embedded SQL run faster than my stored procedures?

A, Recompilations might be the source of the slower stored procedure speed. To find out for sure, you need to do some performance investigation, such as looking at Showplans for each type of query versus calling the stored procedures and comparing query plan cache hits to cache misses. You can also try coding the object owner for referenced tables, views, and procedures inside your stored procedures, as the following example shows: SELECT * FROM dbo.mytable This technique helps you reuse plans and prevent cache misses. Q, How Can I Generate a Series of Random Integers With T-SQL? Here is a stored procedure that will generate a series of random numbers and return them in a result set. This SQL Server stored procedure returns a result set that containing a series of random integers. Inputs are: @begin_no - lowest number to be generated, @end_no - highest number to be generated Calling syntax: Exec sp_GenerateRandomNumberResultSet Begin, End CREATE procedure sp_GenerateRandomNumberResultSet @begin_no int=1, @end_no int=100 As Set nocount on Declare @rn int, @rec int, @cnt int Set @cnt=@end_no-@begin_no+1 Create Table #t (RecNo int identity(1,1), RandNo int null) Set @rec=1 While @rec<=@cnt Begin Insert Into #t (RandNo) Values (null) Set @rec=@rec+1 End Set @rec=1 While @rec<=@cnt Begin Set @rn=rand()*@cnt+@begin_no If @begin_no<0 Set @rn=@rn-1 Update #t Set RandNo=@rn Where RecNo=@rec And @rn Not In (Select RandNo From #t Where RandNo Is Not Null) If @@rowcount>0 Set @rec=@rec+1 End Set nocount off Select RandNo From #t Drop Table #t Go

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Transact-SQL SQL Server Performance Tuning Tips 1.

Slow queries can be caused by a wide variety of reasons. Some include: • Lack of useful indexes • Lack of useful I/O striping • Out-of-date statistics or lack of useful statistics • Lack of physical memory • Slow network connection • Transact-SQL queries transferring large amounts of data from the server to the client • Blocked locks or deadlocks • Performing OLTP and OLAP queries on the same physical server • Poorly designed query Any one or more of the above items can cause slow query performance. Each of these problem areas, and more, are discussed in detail on this page and other pages on this website. [6.5, 7.0, 2000] Updated 9-22-2000

2.

This tip may sound obvious to most of you, but I have seen professional developers, in two major SQL Server-based applications used worldwide, not follow it. And that is to always include a WHERE clause in your SELECT statement to narrow the number of rows returned. If you don't use a WHERE clause, then SQL Server will perform a table scan of your table and return all of the rows. In some case you may want to return all rows, and not using a WHERE clause is appropriate in this case. But if you don't need all the rows returned, use a WHERE clause to limit the number of rows returned. [6.5, 7.0, 2000] Added 12-8-2000

3.

Don't be afraid to make liberal use of in-line and block comments in your Transact-SQL code, they will not affect the performance of your application. [6.5, 7.0, 2000] Added 10-4-2000

4.

If possible, avoid using SQL Server cursors. They generally use a lot of SQL Server resources and reduce the performance and scalability of your applications. If you need to perform row-by-row operations, try to find another method to perform the task. Some options are to perform the task at the client, use tempdb tables, use derived tables, use a correlated sub-query, or use the CASE statement. More often than not, there are non-cursor techniques that can be used to perform the same tasks as a SQL Server cursor. [6.5, 7.0, 2000] Updated 5-18-2001

5.

To help identify long running queries, use the SQL Server Profiler Create Trace Wizard to run the "TSQL By Duration" trace. You can specify the length of the long running queries you are trying to identify (such as over 1000 milliseconds), and then have these recorded in a log for you to investigate later. [7.0, 2000] More info from Microsoft

6.

When using the UNION statement, keep in mind that by default it performs the equivalent of a SELECT DISTINCT on the results set. If you know that there will not be any duplication of rows created as a result of using the UNION statement, then use the UNION ALL statement instead. This variation of the statement does not look for duplicate rows and runs must faster than the UNION, which does look for duplicate rows, whether or not there are any. [6.5, 7.0, 2000]

7.

Carefully evaluate whether your query needs the DISTINCT clause or not. The DISTINCT clause slows down virtually every query it is in. Some developers automatically add this clause to every one of their SELECT statements, even when it is not necessary. This is a bad habit that should be stopped. In addition, keep in mind that in some cases, duplicate results in a query are not a problem. If this is the case, then don't use a DISTINCT clause. [6.5, 7.0, 2000] Added 8-29-2000

8.

In your queries, don't return column data you don't need. For example, you should not use SELECT * to return all the columns from a table if you don't need all the data from each column. [6.5, 7.0, 2000]

9.

In your queries, don't return rows you don't need. Use the WHERE clause to return only those rows you need right now. [6.5, 7.0, 2000]

10. If your users perform many ad hoc queries on your SQL Server data, and you find that many of these "poorly-written" queries take up an excessive amount of SQL Server resources, consider using the "query governor cost limit" configuration option to limit how long a query can run. You can specify

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the maximum amount of "seconds" a query will run, and whenever the query optimizer determines that a particular query will exceed the maximum limit, the query will be aborted before it even begins. Although the value you set for this setting is stated as "seconds", it does not mean seconds like we think of seconds. Instead, it relates to the actual estimated cost of the query as calculated by the query optimizer. You may have to experiment with this value until you find one that meets your needs. There are two ways to set this option. First, you can change it at the server level (all queries running on the server are affected by it) using sp_configure "query governor cost limit", or you can set it at the connection level (only this connection is affected) by using the SET QUERY_GOVERNOR_COST_LIMIT command. [7.0, 2000] Updated 6-5-2001

11. If your application allows users to run queries, but you are unable in your application to easily prevent users from returning hundreds, even thousands of unnecessary rows of data they don't need, consider using the TOP operator within the SELECT statement. This way, you can limit how may rows are returned, even if the user doesn't enter any criteria to help reduce the number or rows returned to the client. For example, the statement: SELECT TOP 100 fname, lname FROM customers WHERE state = 'mo' limits the results to the first 100 rows returned, even if 10,000 rows actually meet the criteria of the WHERE clause. When the specified number of rows is reached, all processing on the query stops, potentially saving SQL Server overhead, and boosting performance. The TOP operator works by specifying a specific number of rows, like the example above, or by specifying a percentage value, like this: SELECT TOP 10 PERCENT fname, lname FROM customers WHERE state = 'mo' In the above example, only 10 percent of the available rows would be returned. [7.0, 2000] Updated 1227-2001

12. You may have heard of a SET command called SET ROWCOUNT. Like the TOP operator, it is designed to limit how many rows are returned from a SELECT statement. In effect, the SET ROWCOUNT and the TOP operator perform the same function. While is some cases, using either option works equally efficiently, there are some instances (such as rows returned from an unsorted heap) where the TOP operator is more efficient than using SET ROWCOUNT. Because of this, using the TOP operator is preferable to using SET ROWCOUNT to limit the number of rows returned by a query. [7.0, 2000] Added 12-27-2001

13. Try to avoid WHERE clauses that are non-sargable. Non-sargable search arguments in the WHERE clause, such as "IS NULL", "OR", "<>", "!=", "!>", "!<", "NOT", "NOT EXISTS", "NOT IN", "NOT LIKE", and "LIKE %500" can prevent the query optimizer from using an index to perform a search. In addition, expressions that include a function on a column, or expressions that have the same column on both sides of the operator, are not sargable. But not every WHERE clause that has a non-sargable expression in it is doomed to a table scan. If the WHERE clause includes both sargable and non-sargable clauses, then at least the sargable clauses can use an index (if one exists) to help access the data quickly. [6.5, 7.0, 2000]

14. If many queries are run off very large tables, consider breaking down the tables into one or more logical subsets using views. This works best for data that is easily partitioned, such as data based on date or location. This way, users can query off the appropriate view instead of the base table, saving a lot of I/O overhead. [6.5, 7.0, 2000]

15. If you application needs to retrieve summary data often, but you don't want to have the overhead of calculating it on the fly when it is needed, consider using a trigger that updates summary values after the initial transaction completes. While the trigger has some overhead, overall, it may be less that having to calculate the data every time the summary data is needed. You will have to decide which method is fastest in your environment. [6.5, 7.0, 2000]

16. When you have a choice of using a constraint or a trigger to perform the same task, always choose the constraint. The same goes if you have the option of using either a constraint or a rule, or a constraint or a default. Constraints require less overhead than triggers, rules, and defaults. [6.5, 7.0, 2000]

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17. Don't implement redundant integrity features in your database. For example, if you are using primary key and foreign key constraints to enforce referential integrity, don't add unnecessary overhead by also adding a trigger that performs the same function. The same goes for using both constraints and defaults or constraints and rules that perform redundant work. While this may sound obvious, it is not uncommon to find in SQL Server databases. [6.5, 7.0, 2000]

18. If you have the choice of using a join or a subquery to perform the same task, generally the join is faster. But this is not always the case, you can may want to test the query using both methods to determine which is faster for your particular application. [6.5, 7.0, 2000]

19. If your application needs to insert a large binary value into an image data column, perform this task using a stored procedure, not using an INSERT statement embedded in your application. The reason for this is because the application must first convert the binary value into a character string (which doubles its size, thus increasing network traffic and taking more time) before it can be sent to the server. And when the server receives the character string, it then has to convert it back to the binary format (taking even more time). Using a stored procedure avoids all this. [6.5, 7.0, 2000]

20. If you need to create a primary key (using a value meaningless to the record, other than providing a unique value for a record), many developers will use either a identity field (with an integer data type) or a uniqueidentifier data type. If your application is not sensitive to either option, then you will most likely want to choose the identity field over the uniqueidentifier field. The reason for this is that the identity field (using the integer data type) only takes up 4 bytes, while the uniqueidentifier field takes 16 bytes. Using the identifier field will create a smaller and faster index. [7.0, 2000] Added 8-5-2000

21. When you have a choice of using the IN or the EXISTS clause in your Transact-SQL, you will generally want to use the EXISTS clause, as it is more efficient and performs faster. [6.5, 7.0, 2000] Added 8-7-2000

22. When you have a choice of using the IN or the BETWEEN clauses in your Transact-SQL, you will generally want to use the BETWEEN clause, as it is much more efficient. For example: SELECT customer_number, customer_name FROM customer WHERE customer_number in (1000, 1001, 1002, 1003, 1004) is much less efficient than this: SELECT customer_number, customer_name FROM customer WHERE customer_number BETWEEN 1000 and 1004 Assuming there is a useful index on customer_number, the Query Optimizer can locate a range of numbers much faster (using BETWEEN) than it can find a series of numbers using the IN clause (which is really just another form of the OR clause). [6.5, 7.0, 2000] Added 12-8-2000

23. If possible, try to avoid using the SUBSTRING function in your WHERE clauses. Depending on how it is constructed, using the SUBSTRING function can force a table scan instead of allowing the optimizer to use an index (assuming there is one). If the substring you are searching for does not include the first character of the column you are searching for, then a table scan is performed. [6.5, 7.0, 2000] Added 8-14-2000

24. Where possible, avoid string concatenation, as it is not a fast process. [6.5, 7.0, 2000] Added 8-152000

25. If possible, try to avoid using data conversion functions in the WHERE clause. If you do, you may be forcing the optimizer to perform a table scan rather than using an applicable index. [6.5, 7.0, 2000] Added 8-14-2000

26. If your application requires you to create temporary tables for use on a global or per connection use, consider the possibility of creating indexes for these temporary tables. While most temporary tables probably won't need, or even can use an index, some larger temporary tables can benefit from them. A properly designed index on a temporary table can be as great a benefit as a properly designed index on a standard database table. [6.5, 7.0, 2000] Added 8-14-2000

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27. Generally, you only want to encapsulate your Transact-SQL code in a transaction if it is going to modify the database. Transactions help to ensure database consistency and are an important tool in the developer's toolbox. But if you put non-database modifying Transact-SQL in a transaction, you are producing unnecessary overhead for your application and SQL Server. For example, Transact-SQL used for creating reports doesn't usually need the benefits (and the overhead) associated with transactions. [6.5, 7.0, 2000] Added 8-14-2000

28. Both the MIN() and MAX() functions can take advantage of indexes on columns. So if you perform these functions often, you might want to add an index to the relevant columns, assuming they don't already exit. [6.5, 7.0, 2000] Added 8-14-2000

29. Generally, avoid using optimizer hints in your queries. This is because it is generally very hard to outguess the Query Optimizer. Optimizer hints are special keywords that you include with your query to force how the Query Optimizer runs. If you decide to include a hint in a query, this forces the Query Optimizer to become static, preventing the Query Optimizer from dynamically adapting to the current environment for the given query. More often than not, this hurts, not helps performance. If you think that a hint might be necessary to optimize your query, be sure you first do all of the following first: • Update the statistics on the relevant tables. • If the problem query is inside a stored procedure, recompile it. • Review the search arguments to see if they are sargable, and if not, try to rewrite them so that they are sargable. • Review the current indexes, and make changes if necessary. If you have done all of the above, and the query is not running as you expect, then you may want to consider using an appropriate optimizer hint. If you haven't heeded my advice and have decided to use some hints, keep in mind that as your data changes, and as the Query Optimizer changes (through service packs and new releases of SQL Server), your hard-coded hints may no longer offer the benefits they once did. So if you use hints, you need to periodically review them to see if they are still performing as expected. [6.5, 7.0, 2000] Updated 3-6-2001

30. If you want to boost the performance of a query that includes an AND operator in the WHERE clause, consider the following: • Of the search criterions in the WHERE clause, at least one of them should be based on a highly selective column that has an index. • If at least one of the search criterions in the WHERE clause is not highly selective, consider adding indexes to all of the columns referenced in the WHERE clause. [7.0, 2000] Added 9-11-2000

31. While views are often convenient to use, especially for restricting users from seeing data they should not see, they aren't good for performance. So if database performance is your goal, avoid using views (SQL Server 2000 Indexed Views are another story). Here's why. When the Query Optimizer gets a request to run a view, it runs it just as if you had run the view's SELECT statement from the Query Analyzer. If fact, a view runs slightly slower than the same SELECT statement run from the Query Analyzer--but you probably would not notice the difference-because of the additional overhead caused by the view. Unlike stored procedures, views offer no preoptimization. Instead of embedding SELECT statements in a view, put them in a stored procedure instead for optimum performance. Not only do you get the added performance boost, you can also use the stored procedure to restrict user access to table columns they should not see. [6.5, 7.0, 2000] Added 5-7-2001

32. Try to avoid nesting views (referring to a view from within a view). While this is not prohibited, it makes it more difficult to identify the source of any performance problems. A better idea is to create separate views instead of nesting them. [6.5, 7.0, 2000] Added 10-9-2000

33. Don't use DISTINCT or ORDER BY in your SELECT statements unless you really need them. Both options can add a lot of additional overhead to your query, and they aren't always needed for your application. [6.5, 7.0, 2000] Added 11-27-2000

34. If your SELECT statement includes an IN option along with a list of values to be tested in the query, order the list of values so that the most frequently found values are placed at the first of the list, and the

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less frequently found values are placed at the end of the list. This can speed performance because the IN option returns true as soon as any of the values in the list produce a match. The sooner the match is made, the faster the query completes. [6.5, 7.0, 2000] Added 11-27-2000

35. If you need to use the SELECT INTO option, keep in mind that it can lock system tables, preventing others users from accessing the data they need. If you do need to use SELECT INTO, try to schedule it when your SQL Server is less busy, and try to keep the amount of data inserted to a minimum. [6.5, 7.0, 2000] Added 11-28-2000

36. If your SELECT statement contains a HAVING clause, write your query so that the WHERE clause does most of the work (removing undesired rows) instead of the HAVING clause do the work of removing undesired rows. Using the WHERE clause appropriately can eliminate unnecessary rows before they get to the GROUP BY and HAVING clause, saving some unnecessary work, and boosting performance. For example, in a SELECT statement with WHERE, GROUP BY, and HAVING clauses, here's what happens. First, the WHERE clause is used to select the appropriate rows that need to be grouped. Next, the GROUP BY clause divides the rows into sets of grouped rows, and then aggregates their values. And last, the HAVING clause then eliminates undesired aggregated groups. If the WHERE clause is used to eliminate as many of the undesired rows as possible, this means the GROUP BY and the HAVING clauses will have less work to do, boosting the overall performance of the query. [6.5, 7.0, 2000] Added 12-11-2000

37. If you need to write a SELECT statement to retrieve data from a single table, don't SELECT the data from a view that points to multiple tables. Instead, SELECT the data from the table directly, or from a view that only contains the table you are interested in. If you SELECT the data from the multi-table view, the query will experience unnecessary overhead, and performance will be hindered. [6.5, 7.0, 2000] Added 12-11-2000

38. If your application performs many wildcard (LIKE %) text searches on CHAR or VARCHAR columns, consider using SQL Server's full-text search option. The Search Service can significantly speed up wildcard searches of text stored in a database. [7.0, 2000] Updated 1-12-2001

39. The GROUP BY clause can be used with or without an aggregate function. But if you want optimum performance, don't use the GROUP BY clause without an aggregate function. This is because you can accomplish the same end result by using the DISTINCT option instead, and it is faster. For example, you could write your query two different ways: USE Northwind SELECT OrderID FROM [Order Details] WHERE UnitPrice > 10 GROUP BY OrderID or USE Northwind SELECT DISTINCT OrderID FROM [Order Details] WHERE UnitPrice > 10 Both of the above queries produce the same results, but the second one will use less resources and perform faster. [6.5, 7.0, 2000] Added 1-12-2001

40. Generally, it is better to perform multiple UPDATEs on records in one fell swoop (using one query), instead of running the UPDATE statement multiple times (using multiple queries). For example, you could accomplish the same goal two different ways: USE Northwind UPDATE Products SET UnitPrice = UnitPrice * 1.06 WHERE UnitPrice > 5 GO USE Northwind UPDATE Products

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SET UnitPrice = ROUND(UnitPrice, 2) WHERE UnitPrice > 5 GO or USE Northwind UPDATE Products SET UnitPrice = ROUND(UnitPrice * 1.06, 2) WHERE UnitPrice > 5 GO As is obvious from this example, the first option requires two queries to accomplish the same task as the second query. Running one query instead of two or more usually produces the best performance. [6.5, 7.0, 2000] Added 1-19-2001.

41. Sometimes perception is more important that reality. For example, which of the following two queries is the fastest: • A query that takes 30 seconds to run, and then displays all of the required results. • A query that takes 60 seconds to run, but displays the first screen full of records in less than 1 second. Most DBAs would choose the first option as it takes less server resources and performs faster. But from many user's point-of-view, the second one may be more palatable. By getting immediate feedback, the user gets the impression that the application is fast, even though in the background, it is not. If you run into situations where perception is more important than raw performance, consider using the FAST query hint. The FAST query hint is used with the SELECT statement using this form: OPTION(FAST number_of_rows) where number_of_rows is the number of rows that are to be displayed as fast as possible. When this hint is added to a SELECT statement, it tells the Query Optimizer to return the specified number of rows as fast as possible, without regard to how long it will take to perform the overall query. Before rolling out an application using this hint, I would suggest you test it thoroughly to see that it performs as you expect. You may find out that the query may take about the same amount of time whether the hint is used or not. If this the case, then don't use the hint. [7.0, 2000] Added 3-6-2001

42. Instead of using temporary tables, consider using a derived table instead. A derived table is the result of using a SELECT statement in the FROM clause of an existing SELECT statement. By using derived tables instead of temporary tables, we can reduce I/O and boost our application's performance. [7.0, 2000] Added 3-9-2001 More info on derived tables.

43. When using the WHILE statement, don't avoid the use of BREAK just because some people consider it bad programming form. Often when creating Transact-SQL code using the WHILE statement, you can avoid using BREAK by moving a few lines of code around. If this works in your case, then by all means don't use BREAK. But if your efforts to avoid using BREAK require you to add additional lines of code that makes your code run slower, then don't do that. Sometimes, using BREAK can speed up the execution of your WHILE statements. [6.5, 7.0, 2000] Added 5-18-2001

44. Computed columns in SQL Server 2000 can be indexed if they meet all of the following criteria: • • • •

The computed column's expression is deterministic. The ANSI_NULL connection-level object was on when the table was created. TEXT, NTEXT, or IMAGE data types are not used in the computed column. The physical connection used to create the index, and all connections used to INSERT, UPDATE, or DELETE rows in the table must have these six SET options properly configured: ANSI_NULLS = ON, ANSI_PADDINGS = ON, ANSI_WARNINGS = ON, ARITHABORT = ON, CONCAT_NULL_YIELDS_NULL = ON, QUOTED_IDENTIFIER = ON, NUMERIC_ROUNDABORT = OFF. [2000] Added 10-9-2000

45. One of the advantages of using SQL Server for two-tier and three-tier applications is that you can offload much (if not most) of the data processing work from the other tiers and place it on

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SQL Server. The more work you can perform within SQL Server, the fewer the network roundtrips that need to be made between the various tiers and SQL Server. And generally the fewer the network roundtrips, the more scalable and faster the application becomes. But in some applications, such as those than involve complex math, SQL Server has traditionally been weak. In these cases, complex math often could not be performed within SQL Server, instead it had to be performed on another tier, causing more network roundtrips than desired. Now that SQL Server 2000 supports user-defined functions (UDFs), this is becoming less of a problem. UDFs allow developers to perform many complex math functions from within SQL Server, functions that previously could only be performed outside of SQL Server. By taking advantage of UDFs, more work can stay with SQL Server instead of being shuttled to another tier, reducing network roundtrips, and potentially boosting your application's performance. Obviously, boosting your application's performance is not as simple as moving math functions to SQL Server, but it is one more new feature of SQL Server 2000 that developers can take advantage of in order to boost their application's scalability and performance. [2000] Added 12-19-2000

46. When creating scalar user-defined functions, avoid as a matter of routine applying them to large result sets. This is because complex user-defined functions have the potential of involving high overhead, and there is no way to know for sure how they will affect performance when dealing with large result sets. Of course, if you know for sure that the user-defined function is not a performance problem, then using it is OK in larger results sets. You will want to test this theory before the userdefined function is put into production. [2000] Added 10-9-2000 47. SQL Server 2000 offers a new data type called "table." Its main purpose is for the temporary storage of a set of rows. A variable, of type "table," behaves as if it is a local variable. And like local variables, it has a limited scope, which is within the batch, function, or stored procedure in which it was declared. In most cases, a table variable can be used like a normal table. SELECTs, INSERTs, UPDATEs, and DELETEs can all be made against a table variable. For best performance, if you need a temporary table in your Transact-SQL code, try to use a table variable instead of creating a conventional temporary table instead. Table variables are created and manipulated in memory instead of the tempdb database, making them much faster. In addition, table variables found in stored procedures result in fewer compilations (than when using temporary tables), and transactions using table variables only last as long as the duration of an update on the table variable, requiring less locking and logging resources. [2000] Added 8-7-2001

48. Don't repeatedly reuse the same function to calculate the same result over and over within your Transact-SQL code. For example, if you need to reuse the value of the length of a string over and over within your code, perform the LEN function once on the string, and this assign the result to a variable, and then use this variable, over and over, as needed in your code. Don't recalculate the same value over and over again by reusing the LEN function each time you need the value, as it wastes SQL Server resources and hurts performance. [6.5, 7.0, 2000] Added 8-9-2001

49. Most of you are probably familiar with the aggregate SUM() function and how it works. Occasionally, it would be nice if SQL Server had a PRODUCT() function, which it does not. While SUM() is used to sum a group a data, the theoretical PRODUCT() function would find the product of a group of data. One way around the problem of there not being a PRODUCT() function in SQL Server is to use some combination of a cursor and/or temporary tables. As you can imagine, this would not be very efficient. A better choice would be to use a set-based function, like the theoretical PRODUCT() function. With a little algebra, you can simulate a PRODUCT() function in SQL Server using the built-in SQL Server LOG10(), POWER(), and SUM() function working together. This is because logarithms allow you to find the product of numbers by summing them. This was how the products of large numbers were found before the days of calculators. (Are you old enough to remember using logarithm tables in school? I am. Ouch!) Below is a very simple example of how you can use a combination of the LOG10(), POWER(), and SUM() functions in SQL Server to simulate a PRODUCT() function. You will probably want to modify it to meet your specific needs, such as to eliminate null data, zero data, or data that might be negative. SELECT column_name1, POWER(10,SUM(LOG10(column_name2))) AS Product FROM table_name GROUP BY column_name1 For example, let's look at the following to see how this works. Record 1 (1000, 2) Record 2 (1000, 2) Record 3 (1000, 2)

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Record 4 (1001, 3) Record 5 (1001, 3) Our goal here is find the product of all the records where column_name1 = 1000 and to find the product of all the records where column_name_name1 = 1001. When the above query is run, we get these results: 1000, 8 1001, 9 What has happened is that where column_name1 = 1000 (which are the first three records in our sample data), the values in column_name2 (which are 2 and 2 and 2) are multiplied together to return 8. In addition, where column_name1 = 1001 (which are the last two records in our sample data), the values in column_name2 (which are 3 and 3) are multiplied together to return 9. Creating your own PRODUCT() function produces much faster results than trying to accomplish the same task by using a cursor and/or temporary tables. [6.5, 7.0, 2000] Added 10-11-2001

50. Many developers choose to use an identify column at their primary key. By design, an identity column does not guarantee that that each newly created row will be consecutively numbered. This means they will most likely be occasional gaps in the identity column numbering scheme. For most applications, occasional gaps in the identity column present no problems. On the other hand, some developers don't like these occasional gaps, trying to avoid them. With some clever use of INSTEAD OF triggers in SQL Server 2000, it is possible prevent these numbering gaps. But at what cost? The problem with trying to force an identify column to number consecutively without gaps can lead to locking and scalability problems, hurting performance. So the recommendation is not to try to get around the identify column's built-in method of working. If you do, expect performance problems. [2000] Added 10-17-2001

51. Avoid using variables in the WHERE clause of a query located in a batch file. Let's find out why this may not be a good idea. First, let's look at the following code: SELECT employee_id FROM employees WHERE age = 30 and service_years = 10 Assuming that both the age and the service_years columns have indexes, and the table has many thousands of records, then SQL Server's Query Optimizer will select the indexes to perform the query and return results very quickly. Now, let's look at the same query, but written to be more generic, one that you might find in a generic batch file: DECLARE @age int SET @age = "30" DECLARE @service_years int SET @service_years = "10" SELECT employee_id FROM employees WHERE age = @age and service_years = @service_years When the above code is run, even though both the age and the service_years columns have indexes, they won't be used, and a table scan will be used instead, potentially greatly increasing the amount of time for the query to run. The reason the indexes are not used is because the Query Analyzer does not know the value of the variables when it selects an access method to perform the query. Because this is a batch file, only one pass is made of the Transact-SQL code, preventing the Query Optimizer from knowing what it needs to know in order to select an access method that uses the indexes. If you cannot avoid using variables in the WHERE clauses of batch scripts, consider using an INDEX query hint to tell the Query Optimizer to use the available indexes instead of ignoring them and performing a table scan. This of course that the indexes are highly selective. If the indexes are not highly selective, then a table scan most likely be more efficient than using the available indexes. [6.5, 7.0, 2000] Added 12-7-2001

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Performance Tuning Tips for Creating Visual Basic Applications Using SQL Server 52. While ADO (and other VB object libraries) make database manipulation easy for the programmer, using these shortcuts can kill SQL Server performance. As a rule of thumb, encapsulate your DML (Data Manipulation Language) in stored procedures and run them from your VB application. This bypasses object library overhead (such as reducing cursors) and reduces the chatter between the VB application and SQL Server over the network. So what does this mean in practice? Essentially, avoid using the ADO recordset object to modify (INSERT, UPDATE, DELETE) data in your VB code. Instead, use Transact-SQL, encapsulated in stored procedures, to modify data in a SQL Server database. An ADO recordset should be used as a method of reading data, not modifying data. [6.5, 7.0, 2000] Updated 9-12-2001

53. When using an ADO recordset to return data from SQL Server, the most efficient way is to use what is often called a firehose cursor. The firehouse cursor is really an incorrect term because it is not a cursor. A firehose cursor is just a method to quickly move data from SQL Server to the client that requested it. Essentially, a firehose cursor sends the requested data (from the query) to an output buffer on SQL Server. Once the output buffer is full, it waits until the client can retrieve the data from the output buffer. Then the output buffer is filled again. This process repeats over and over until all of the data is sent to the client. Another advantage of this method is that records are only locked long enough to be moved to the output buffer. When you open an ADO RecordSet and use its default settings, a firehose cursor is automatically used by default. If you want to specify a firehouse cursor manually, you can do so by using these property settings: • CursorType = adForwardOnly • CursorLocation = adUseServer • LockType = adLockReadOnly • CacheSize = 1 When the client receives the data from the firehose cursor, the data should be read into a local data structure for local use by the client. [6.5, 7.0, 2000] Updated 9-12-2001

54. When accessing data on a SQL Server, write your VB code so as to minimize the number of round-trips between the application and SQL Server. Each and every time you use ADO to execute Transact-SQL code to get data from SQL Server, multiple, time-consuming steps have to occur. For example: • Your VB code must generate a request to SQL Server in the form of a Transact-SQL statement. • The statement is sent to the database through the Connection object. • The request from the Connection object has to be translated into packets that can be sent over the network. • The packets move over the network. • When the packets arrive at SQL Server, they must be converted back into a form useable by SQL Server. • SQL Server must then process the Transact-SQL statement. Assuming a stored procedure is not used, then this code must be optimized and compiled, then executed. • The results, in the form of TDS (Tabular Data Stream), are then translated into packets that can be sent over the network. • The packets move over the network, again. • When the packets arrive at SQL Server, they must be converted back into TDS format. • When ADO received the TDS data, it is converted into a recordset, ready to be used by the application. If you know much about the technical details of networking, then you know that the above steps have been oversimplified. The point to remember is that round-trips between your application and SQL Server are expensive in time and resources, and you need to do your best in your code to minimize them. [6.5, 7.0, 2000] 7-19-01

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55. One way to help reduce round-trips between your application and SQL Server is to move the data you need at the client from SQL Server in a single query, not in multiple queries. I have seen some applications that only retrieve one row at a time, making a round-trip for every row needed by the application. This can be very expensive in resources and it hurts performance. Of course, you can't always know what rows will be needed ahead of time, but the better you can guess, even if you guess and return too many rows, returning them in one round-trip is usually more efficient than retrieving only one row at a time. [6.5, 7.0, 2000] 7-19-01

56. When retrieving data from a SQL Server 7 database, take full advantage of views when appropriate. This is especially true if you are not encapsulating your Transact-SQL in stored procedures as recommended. While calling a view is not usually as efficient as using a stored procedure to retrieve data, it is much more efficient that using embedded Transact-SQL in your ASP code or COM components. [6.5, 7.0, 2000]

57. Don't use DAO to access SQL Server, it is performance suicide. Also avoid ODBCDirect. Instead, use RDO or ADO. [6.5, 7.0, 2000]

58. When creating a connection using ADO, be sure you use the OLE DB provider, not the older ODBC provider for SQL Server, or the ODBC provider for OLE DB. The parameter you will use in your connection string is "provider=sqloledb". The OLE DB provider performs much more efficiently than the ODBC provider, providing better performance. [7.0, 2000] 7-19-2001

59. Use stored procedures instead of embedding Transact-SQL in your VB code. This significantly reduces network traffic and speeds up query execution. [6.5, 7.0, 2000]

60. If you are VB developer and need to access SQL Server data, but don't have the time or interest in learning how to write stored procedures, consider using the GetRows method of the RecordSet object. The GetRows method is used to pull all the records from the recordset into an array, which is much faster than using embedded Transact-SQL to download a RecordSet to your application. [6.5, 7.0, 2000]

61. If possible in your application, use stored procedures to "batch" a set of related Transact-SQL statements together, instead of calling a separate stored procedure for every database task you want to perform. This helps to reduce network traffic and server overhead. [6.5, 7.0, 2000]

62. If you have a related group, or batch, or Transact-SQL statements you want to execute, but you don't want to use a stored procedure, as generally recommended for dealing with batches of Transact-SQL statements, one option you can use to boost performance if your VB code is to concatenate two or more separate Transact-SQL statements into a single batch and execute them as a single message. This is much more efficient that sending the Transact-SQL code to SQL Server as many different messages. [6.5, 7.0, 2000] Added 7-19-01

63. When SELECTing data from your application to be returned to it from SQL Server, limit the amount of rows returned to only those that are needed now. If necessary, force the user to enter selection criteria to limit the results set. [6.5, 7.0, 2000]

64. If your application allows users to run queries, but you are unable in your application to easily prevent users from returning hundreds, even thousands of unnecessary rows of data they don't need, consider using the TOP operator within the query. This way, you can limit how may rows are returned, even if the user doesn't enter any criteria to help reduce the number or rows returned to the client. [6.5, 7.0, 2000]

65. If your application needs to perform looping, try to put the loop inside a stored procedure so it can be executed on the server without having to make round trips between the client and server. [6.5, 7.0, 2000]

66. When creating transactions in your application, don't create them using ADO's methods. Instead, encapsulate the transaction in stored procedure so that it executes on the server. [6.5, 7.0, 2000]

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67. Keep as much of the business logic of your application as possible off the client. In two-tier designs, put the business logic in stored procedures on the server. In n-tier designs, put the business logic in components on MTS. [6.5, 7.0, 2000]

68. If you have the need to filter or sort data on-the-fly at the client, let ADO do this for you at the client. When the data is first requested by the client from the server (ideally using a stored procedure), have all the data the client wants to "play" with sent to the client. Once the recordset is at the client, then ADO methods can be used to filter or sort the data. This helps to reduce network traffic and takes some of the load off of the server. [6.5, 7.0, 2000]

69. By default, the CacheSize property of the ADO Recordset object determines how many rows are fetched from a server-side cursor at a time. The default is one. This means each row of the recordset is returned one at a time from the server to the client. This is very inefficient. The CacheSize property needs to be set to a much higher figure, such as between 100 and 500, depending on the number of rows that are to be eventually returned from the server to the client. [6.5, 7.0, 2000]

70. When calling SQL Server stored procedures from the ADO Command object, don't use the Refresh method to identify the parameters of a stored procedure. This produces extra network traffic and slows performance. Instead, explicitly create the parameters yourself using ADO code. [7.0, 2000]

71. ADO allows you to create four different types of SQL Server cursors. Each has its own place, and you will want to choose the cursor that uses the least possible resources for the task at hand. When at all possible, attempt to use the Forward-Only cursor, which uses the least amount of overhead of the four cursor types. [6.5, 7.0, 2000]

72. Avoid using the MoveFirst method of the RecordSet object when using a Forward-Only cursor. In effect, when you use this method, it re-executes the entire query and repopulates the Forward-Only cursor, increasing server overhead. [6.5, 7.0, 2000] Added 9-12-2001

73. If you create COM objects to encapsulate database access, try to follow these two suggestions if you want optimum speed: 1) use in-process dlls; and 2) use early-binding. [6.5, 7.0, 2000]

74. Consider using ADO's ability to create disconnected recordsets to help reduce the load on SQL Server. [6.5, 7.0, 2000]

75. When storing your SQL Server data into VB variables, always use strongly typed variables. Avoid using the variant data type (which is not always possible), as it has greater overhead than the other data types. [6.5, 7.0, 2000]

76. If you create object variables in your VB code to refer to COM objects that hold SQL Server data, be sure to strongly type them. Avoid using the AS OBJECT keywords, instead, always explicitly specify the type of object you want to create. [6.5, 7.0, 2000]

77. When instantiating COM objects to hold SQL Server data, create them explicitly, not implicitly. [6.5, 7.0, 2000]

78. If you will be calling the same stored procedure, view, or SQL statements over and over again in your code, don't create a new Command object each time. Instead, reuse the Command object. [6.5, 7.0, 2000]

79. When looping through recordsets, be sure you bind columns to field objects before the looping begins. Don't use the Fields collection of the Recordset object to assign values for fields in a Recordset within each loop, it incurs much more overhead. [6.5, 7.0, 2000]

80. If you know that the results of a query from within a stored procedure you call will return only one row of data (and not an entire recordset), don't open an ADO Recordset for the purpose of retrieving the data. Instead, use a stored procedure output parameter. [6.5, 7.0, 2000]

81. If your application needs to insert a large binary value into an image data column, perform this task using a stored procedure, not using an INSERT statement embedded in your application. The reason

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for this is because the application must first convert the binary value into a character string (which doubles its size, thus increasing network traffic and taking more time) before it can be sent to the server. And when the server receives the character string, it then has to convert it back to the binary format (taking even more time). Using a stored procedure avoids all this. [6.5, 7.0, 2000]

82. When ADO is used to open more than one ForwardOnly recordset on a single Connection object at a time, only the first recordset is opened using the Connection object you previously created. Additional new connections don't use the same Connection object. Instead, separate connections are created for each ForwardOnly recordset you create after the first. This occurs because SQL Server can only open one ForwardOnly cursor per connection. The more connections you create, the greater the stress on SQL Server and performance and scalability suffer. To avoid this problem, don't use a ForwardOnly recordset. Static, Keyset, and Dynamic recordsets don't have this problem. Another option is to use a client side cursor instead of SQL Server cursor. Or you can close each recordset before opening another on the same connection. [6.5, 7.0, 2000]

83. When making your connection to SQL Server, choose DSN-less connections for the fastest connection. Not only does it make database connections faster, it allows you to use the OLE DB provider, which is not available using a DSN-based connection. The OLE DB provider is the fastest provider you can use to access SQL Server. If you do need to use a DSN, select System DSNs over File DSNs, because they are faster when making connections. [7.0, 2000] Updated 7-27-2001

84. When creating a Connection object, always create it explicitly, not implicitly. Opening a Connection object explicitly consumes less resources than opening it implicitly, and it also allows you to more efficiently manage multiple connections and to reassign the various roles that the Connections objects perform within your application. [6.5, 7.0, 2000] Added 12-14-2000

85. When using recordsets, be sure to open them explicitly, not implicitly. When recordsets are opened implicitly, you cannot control the default cursor and lock types, which are, respectively, forwardonly and read-only. If you always open your recordsets explicitly, then you can specify which cursor and lock types you want to invoke for this particular situation, specifying the types with the least amount of overhead to accomplish the task at hand. [6.5, 7.0, 2000] Added 12-14-2000

86. When using ADO to make connections to SQL Server, always be sure you explicitly close any Connection, Recordset, or Command objects you have opened. While letting an object go out of scope will in affect close the object, it is not the same as explicitly closing an object. By explicitly closing these objects and setting them to nothing, you do two things. First, you remove the object sooner than later, helping to free up resources. Second, you eliminate the possibility of "connection creep". Connection creep occurs when connection or resource pooling is used and when connections are not properly closed and released from the pool. This helps to defeat the purpose of pooling and reduces SQL Server's performance. [6.5, 7.0, 2000]

87. If you are connecting to SQL Server via either OLE DB (version 2.0 or higher) or ODBC (version 3.0 or higher), SQL Server connection pooling is automatically implemented for you. Because of this, you don't have to write special code to implement connection pooling yourself. In addition, you don't want to even reuse an ADO connection object, which is commonly done by many VB developers. If you want to take the best advantage of database connection pooling, and optimize your VB application's SQL Server data access, the best advice you can receive is to be sure that you only open a database connection just before you need it, and then close it immediately after you are done with it. Don't leave database connections open if you are not using them. When you create or tear down a database connection in your VB code, you aren't really creating a new connection or tearing down a current connection. What is happening is that your connection requests are send to OLE DB or ODBC, and they determine if a connection needs to be created or torn down. If a new connection is needed, then one is created, or one is used from the current connection pool. And if you request that a connection be torn down, it will actually pool the unused connection until it is needed, or tear it down if it is not reused within a given time period. [6.5, 7.0, 2000] Updated 8-28-2001

88. In order for connection pooling to work correctly, be sure each connection you open uses the same ConnectionString parameters. Connection pooling only works if all of the parameters for the ConnectionString are identical. If they are all not identical, then a new connection will be opened, circumventing connection pooling. [6.5, 7.0, 2000] Added 2-5-2001

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89. If appropriate for your application, locate the application's data access components on the SQL Server where the data is, instead of at the client. This can significantly reduce network traffic and overhead and boost data throughput. [6.5, 7.0, 2000]

90. When creating a Command object to execute a stored procedure against SQL Server, you can use either the adCmdText or the adCmdStoredProc property to tell ADO that you want to execute a stored procedure. Always use the adCmdStoredProc property, which uses RPCs between the client and SQL Server. This acts to bypass parameter translation and boosts performance from 20 to 30 percent over using the adCmdText property. [6.5, 7.0, 2000]

91. If you need to execute a stored procedure from a Command object, and if the stored procedure will not return any rows, you can boost performance of the Command object by setting the adExecuteNoRecords option. This tells the Command object to not ask for a returning rowset, which saves a little overhead and reduce memory usage. [6.5, 7.0, 2000]

92. If you need your VB application to generate a unique value for use in a primary key column in a SQL Server table, performance will be slightly better if you let SQL Server, instead of your VB application, create the unique value. SQL Server can generate unique keys using either an Identity (using the Integer data type) column or by using the NEWID function in a UniqueIdentifier column. Of these two, Identify columns offer better performance. [6.5, 7.0, 2000]

93. When creating COM components to access SQL Server, try to design the component to have as few properties as possible. For example, instead of having a property for every column of data you want to send back or forth between the database and your application, create one generic property that can be used to send all of the columns at one time. What this does is reduce the number of calls that must be made by the component, reducing overhead on the component and SQL Server. [6.5, 7.0, 2000]

94. When setting Connection Object properties, use the following dot notation instead of using the fully qualified object property notation, whenever appropriate, as it is faster. Use this format for optimum speed: WITH cn .ConnectionTimeout = 100 .ConnectionString = "xyz" .Cursor Location = adUseClient END cn Not this format: cn.ConnectionTimeout = 100 cn.ConnectionString = "xyz" cn.Cursor Location = adUseClient If you are one of those people who need to prove this claim for yourself, try running the following code: Public Declare Function GetTickCount Lib "kernel32" () As Long Sub T1() Dim i As Long Dim cn As ADODB.Connection Dim Fast As Long Dim Slow As Long Set cn = New ADODB.Connection With cn .ConnectionTimeout = 100 .ConnectionString = "xyz" .CursorLocation = adUseClient End With cn.ConnectionTimeout = 100 cn.ConnectionString = "xyz" cn.CursorLocation = adUseClient

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For i = 1 To 100000 Fast = Fast - GetTickCount With cn .ConnectionTimeout = 100 .ConnectionString = "xyz" .CursorLocation = adUseClient End With Fast = Fast + GetTickCount Slow = Slow - GetTickCount cn.ConnectionTimeout = 100 cn.ConnectionString = "xyz" cn.CursorLocation = adUseClient Slow = Slow + GetTickCount Next MsgBox "Fast=" & Fast & vbCrLf & "Slow=" & Slow End Sub Thanks to Gareth Edwards for the above example code. [6.5, 7.0, 2000] Updated 4-5-2001

95. Don't use VB objects to act as a data holder (to store data) in your SQL Server-based applications. Instead, use an array or a collection of user-defined types (UDTs). While using objects you create to store data can be convenient, it also creates a lot of unnecessary overhead. Each time you have to instantiate and then destroy an object hurts performance and scalability. How do you know if an object you have created is storing data that should be stored elsewhere? If the class has mostly properties and few if any methods, then this is a good clue. [6.5, 7.0, 2000] Added 10-26-2000

96. Don't instantiate, initialize, use, and then destroy an object within a loop. If the loop repeats itself much at all, you create an inordinate amount of overhead for your application. Instead, reuse the same object in the loop. One of the best ways to do this is to include a reinitialize method for the object that can be called from within the loop. [6.5, 7.0, 2000] Added 10-26-2000

97. For maximum performance, consolidate most, if not all, of your COM components in a single DLL. COM components instantiated from a single DLL instantiate faster than if they are called from multiple DLLs. [6.5, 7.0, 2000] Added 10-26-2000

98. If you decide not to use a stored procedure to access SQL Server, but instead choose to use an embedded SQL statement in your VB code, and if that embedded SQL statement will be repeated, such as in a loop, consider setting the ADO Command object's "Prepared" property to "True". What this property does is to tell SQL Server to compile and save a copy of your SQL statement in SQL Server's cache. The first time the SQL statement is executed, the statement has to be compiled and stored in memory. But in subsequent calls, the statement is called from the cache, boosting performance because it does not have to be recompiled each time it is called. If the SQL statement will only be executed once, then don't set this option, as it will actually decrease performance for SQL statements that are run only once. The performance boost only comes if the SQL statement is run multiple times. [7.0, 2000] Added 12-27-2000

99. If you use collections in your VB 6 or ASP code, consider instead using dictionaries in order to boost the performance of your application. While you are probably familiar with what and how collections are used, you may not be familiar with dictionaries. The dictionary class (the Scripting.Dictionary Object) is part of the Microsoft Scripting Runtime library, which is delivered with the VB 6 and ASP development environments. Like collections, dictionaries can hold any type of data, and items can be retrieved by using a key, or iterated using the "For Each" syntax. But dictionaries are different in that offer properties and methods not available for collections. And the biggest difference is that they are much faster, about twice as fast. If you haven't learned about dictionaries yet, you need to take the time now to learn about their numerous advantages. [6.5, 7.0, 2000] Added 12-27-2000

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100. To speed up string manipulation in VB (never a fast performing task), consider the following suggestions that can speed string manipulation performance: • When feasible, use fixed-length strings instead of variable-length strings. • Try to minimize string concatenation. • Try to minimize string comparisons. • Try to avoid calculating the length of a string more than once. If you need this data more than once, and the string length does not change, then calculate the length once and store this value in a variable for reuse. • When passing a string to an in-process function, try to pass it by reference instead of by value. • Most VB string functions have two forms, one that produces a variant and one that produces a string (generally has a "$" after the function name). The version that produces the string is faster, and should generally be used. • Consider using byte arrays instead of strings. [6.5, 7.0, 2000] Added 12-28-2000

101. If you still have any legacy VB applications that still use VB-SQL to access SQL Server, you may want to consider rewriting the app. VB-SQL not only provides slow access, it is no longer supported by Microsoft. [6.5] Added 1-2-2001

102. If you are the sort of VB developer who likes to design their applications around objects, you want to keep in mind that over-encapsulating data access within objects can hurt performance. For example, from an OO design approach, you might consider encapsulating data access to each individual table in a SQL Server database, creating a separate class for each table. While this may appeal to your OO design goals, it is inefficient from a performance perspective. Too much encapsulation can lead to situations where you don't take advantage of SQL Server's built-in optimization abilities, it causes too many round-trips to the database, and it can use more database connections than absolutely required. Instead of over-encapsulating your data access in class, a more efficient approach is to use stored procedures to encapsulate your business logic. Stored procedures eliminate these three drawbacks. [6.5, 7.0, 2000] Added 7-19-2001

103. If you use send Transact-SQL code as part of your VB ADO code directly to SQL Server, without using a stored procedure (which we don't recommend if you want best performance, use a stored procedure instead) you want ADO to execute the Transact-SQL code using the sp_executesql system stored procedure, not for ADO to create a temporary stored procedure in one step, execute the temporary stored procedure in the second step, and then drop the temporary stored procedure in a third step. As you can imagine, this is a total of three crosses of the network, and it can greatly increase overhead and hurt performance. How do you know if your ADO code is behaving well? Use the SQL Server Profiler to trace the activity between your VP application and SQL Server. If you see that SQL Server is creating temporary stored procedures, and not using sp_executesql, then you need to review your ADO code, looking for ways to optimize it. [7.0, 2000] Added 8-31-2001

104. Limit the amount of rows you return from a database to populate a pick-list or drop-down box. Lots of rows not only slows down your application, it also makes it less convenient for your user to select the item or items they need. Have you ever had to select from over 100 choices? It is not easy. If you need to give your user a lot of choices, instead of displaying them in one large pick-list or dropdown list, provide a way for the user to filter out any options that are not applicable to them. For the best performance, perform the filtering at the client, not the SQL Server. Ideally, you should use a stored procedure to retrieve the minimum amount of rows you need, then if there are still a lot of rows to to deal with (from the user's perspective), provide a mechanism for the user to filter the list using the various ADO methods available to use for local filtering. This reduces the number of round trips from the client to SQL Server, helping to boost performance. [6.5, 7.0, 2000] Added 12-11-2001

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Miscellaneous SQL Server Performance Tuning Tips 1.

If you need to delete all the rows in a table, don't use DELETE to delete them all, as the DELETE statement is a logged operation and can take time. To perform the same task much faster, use the TRUNCATE TABLE instead, which is not a logged operation. Besides deleting all of the records in a table, this command will also reset the seed of any IDENTITY column back to its original value. [6.5, 7.0, 2000] Updated 7-3-2001

2.

Don't run a screensaver on your production SQL Server, it can unnecessarily use CPU cycles that should be going to your application. The only exception to this is the "blank screen" screensaver, which is OK to use. [6.5, 7.0, 2000]

3.

Use sp_who and sp_who2 (sp_who2 is not documented in the SQL Server Books Online, but offers more details than sp_who) to provide locking and performance-related information about current connections to SQL Server. [6.5, 7.0, 2000]

4.

By default, you cannot use a UNC (Universal Naming Convention) name to specify a location of where to store a SQL Server database or log file. Instead, you must specify a drive letter that refers to a local physical drive or array. But what if you want to store your database or log file on another NT Server or a Network Appliance, Inc. storage system? You can, but you will have to set Trace Flag 1807 on your SQL Server to allow the use of UNC names. [7.0]

5.

For a quick and dirty way to check to see if your SQL Server has maxed out its memory (and causing your server to page), try this. Bring up the Task Manager and go to the "Performance" tab. Here, check out two numbers: the "Total" under "Commit Charge (k)" and the "Total" under "Physical Memory (k)". If the "Total" under "Commit Charge (k)" is greater than the "Total" under "Physical Memory (k)", then your server does not have enough physical memory to run efficiently as it is currently configured and is most likely causing your server to page unnecessarily. Excess paging will slow down your server's performance. If you notice this problem, you will probably want to use the Performance Monitor to further investigate the cause of this problem. You will also want to check to see how much physical memory has been allocated to SQL Server. Most likely, this setting has been set incorrectly, and SQL Server has been set to use too much physical memory. Ideally, SQL Server should be set to allocate physical RAM dynamically. [6.5, 7.0, 2000]

6.

Internet Information Server (IIS) has the ability to send its log files directly to SQL Server for storage. Busy IIS servers can actually get bogged down trying to write log information directly to SQL Server, and because of this, it is generally not recommend to write a web logging information to SQL Server. Instead, logs should be written to text files, and later imported into SQL Server using BCP or DTS. [6.5, 7.0, 2000]

7.

SQL Server 7 has a database compatibility mode that allows applications written for previous versions of SQL Server to run under SQL Server 7. In you want maximum performance for your database, you don't want to run your database in compatibility mode. Instead, it should be running in native SQL Server 7 mode. Of course, this may require you to modify your application to make it SQL Server 7 compliant, but in most cases, the additional work required to update your application will be more than paid for with improved performance. [7.0, 2000]

8.

When experimenting with the tuning of your SQL Server, you may want to run the DBCC DROPCLEANBUFFERS command to remove all the test data from SQL Server's data cache (buffer) between tests to ensure fair testing. If you want to clear out the stored procedure cache, use this command, DBCC FREEPROCCACHE. Both of these commands are for testing purposes and should not be run on a production SQL Server. [7.0, 2000]

9.

Orphan SQL Server sessions can negatively affect SQL Server's performance. An orphan SQL Server session can occur when a client improperly disconnects from SQL Server, such as when the client looses power. When this happens, the client cannot tell SQL Server to properly close the connection, so the SQL Server connection remains open, even though it is not being used. This can affect SQL Server's performance two ways. First, they use up SQL Server connections, which takes up server resources. Secondly, it is possible that the orphan connections may be holding locks that block

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other users, or temp tables or cursors may be held open that also take up unnecessary server resources. NT Server periodically checks for inactive SQL Server sessions, and if it finds any, it will notify SQL Server so that the connection can be removed. Unfortunately, NT Server only performs this check every 1-2 hours, depending on the protocol used. If orphaned SQL Server sessions become a problem, NT Server's registry can be modified so that it checks more often for orphaned connections. Identifying an orphaned connection from SQL Server is very difficult, but if you can identify it, it can be removed by KILLing it using Enterprise Manager or Query Analyzer. [6.5, 7.0, 2000] Added 10-2-2000

10. If your application opens a DB-Library connection to SQL Server 7.0 and it uses either the TCP/IP or IPX/SPX network libraries, and you are using SQL Server 7.0 with no service pack or Service Pack 1, then SQL Server will experience severe memory leaks each time a new connection is opened, that can significantly affect its performance. To resolve this problem, the best solution is to install Service Pack 2, which will correct the problem. If you cannot install the service pack, then another option is to use another network library, such as Named Pipes or Multiprotocol. [7.0] More from Microsoft Added 11-15-2000

11. For best performance, don't mix production databases and development (test or staging) databases on the same physical server. This not only serves to better separate the two functions (production and development), but prevents developers from using up server resources that could be better used by production users. [6.5, 7.0, 2000] Added 11-20-2000 12. When we think of performance, we usually think about speeding up our application's performance. But another way to look at performance is to look at our performance as DBA's or Transact-SQL developers. For example, one of the easiest ways to speed up our Transact-SQL coding, in addition to maintaining and troubleshooting our code once it is written, it to format it in an easy to read format. While there are many different code formatting guidelines available, here are some basic ones you should consider following, if you aren't doing so already: • Begin each line of your Transact-SQL code with a SQL verb, and capitalize all Transact-SQL statements and clauses, such as: SELECT customer_number, customer_name FROM customer WHERE customer_number > 1000 ORDER BY customer_number •

If a line of Transact-SQL code is too long to fit onto one line, indent the following line(s), such as:

SELECT customer_number, customer_name, customer_address, customer_state, customer_zip, customer_phonenumber •

Separate logical groupings of Transact-SQL code by using appropriate comments and documentation explaining what each grouping goes. These are just a few of the many possible guidelines you can follow when writing your Transact-SQL code to make it more readable by you and others. You just need to decide on some standard, and then always follow it in your coding. If you do this, you will definitely boost your coding performance. [6.5, 7.0, 2000] Added 12-5-2000

13. Be wary of allowing users to directly access your databases (especially OLTP databases) with third-party database access tools, such as Microsoft Excel or Access. Many of these tools can wreck havoc with your database's performance. Here are some reasons why: • Often these users aren't experienced using these tools, and create overly complex queries that eat up server resources. At the other extreme, their queries may not be complex enough (such as lacking effective WHERE clauses) and return thousands, if not millions, or unnecessary rows of data. • This reporting activity can often lock rows, pages, or tables, creating user contention for data and reducing the database's performance. • These tools are often file-based. This means that even if an effective query is written, the entire table (or multiple tables in the case of joins) have to be returned to the client software where the query is actually performed, not at the server. This can not only lead to excess server activity, but it can play havoc on your network.

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If you have no choice but to allow users access to your data, try to avoid them hitting your production OLTP databases. Instead, point them to a "reporting" server that has been replicated, or is in the form of a datamart or data warehouse. [6.5, 7.0, 2000] Added 4-18-2001

14. SQL Server 2000 offers support of SSL encryption between clients and the server. While selecting this option prevents the data from being viewed, it also adds additional overhead and reduces performance. Only use SSL encryption if absolutely required. If you need to use SSL encryption, consider purchasing a SSL encryption processor for the server to speed performance. [2000] Added 9-21-2000

15. SQL Server 2000 supports named instances of SQL Server. For example, this feature allows you to run both SQL Server 6.5 and SQL Server 2000 on the same server; or to run SQL Server 7.0 and SQL Server 2000 on the same server; or to run SQL Server 6.5, SQL Server 7.0, and SQL Server 2000 on the same server; and to run up to 16 concurrent instances of SQL Server 2000 on the same server. As you might imagine, each running instance of SQL Server takes up server resources. Although some resources are shared by multiple running instances, such as MSDTC and the Microsoft Search services, most are not. Because of this, each additional instance of SQL Server running on the same server have to fight for available resources, hurting performance. For best performance, run only a single instance (usually the default) on a single physical server. The main reasons for using named instances is for upgrading older versions of SQL Server to SQL Server 2000, transition periods where you need to test your applications on multiple versions of SQL Server, and for use on developer's workstations. [2000] Added 11-14-2000

16. If you run the ALTER TABLE DROP COLUMN statement to drop a variable length or text column, did you know that SQL Server will not automatically reclaim this space after performing this action. To reclaim this space, which will help to reduce unnecessary I/O due to the wasted space, you can run the following command, which is new to SQL Server 2000. DBCC CLEANTABLE (database_name, table_name) Before running this command, you will want to read about it in Books Online to learn about some of its options that may be important to you. [2000] Added 2-28-2001

17. Trace flags, which are used to enable and disable some special database functions temporarily, can often chew up CPU utilization and other resources on your SQL Server unnecessarily. If you just use them for a short time to help diagnose a problem, for example, and then turn them off as soon as your are done using them, then the performance hit you experience is small and temporary. What happens sometimes is that you, or another DBA, turns on a trace flag, but forgets to turn it off. This of course, can negatively affect your SQL Server's performance. If you want to check to see if there are any trace flags turned on on a SQL Server, run this command in Query Analyzer: DBCC TRACESTATUS(-1) If there are any trace flags on, you will see them listed on the screen after running this command. DBCC TRACESTATUS only finds traces created at the client (connection) level. If a trace has been turned on for an entire server, this will not show up. If you find any, you can turn them off using this command: DBCC TRACEOFF(number of trace) [7.0, 2000] Added 6-6-2001

18. SQL Server offers a feature called the black box. When enabled, the black box creates a trace file of the last 128K worth of queries and exception errors. This can be a great tool for troubleshooting some SQL Server problems, such as crashes. Unfortunately, this feature uses up SQL Server resources to maintain the trace file than can negatively affect its performance. Generally, you will want to only turn the black box on when troubleshooting, and turn it off during normal production. This way, your SQL Server will be minimally affected. [7.0, 2000] Added 6-6-2001

19. If you have ever performed a SELECT COUNT(*) on a very large table, you know how long it can take. For example, when I ran the following command on a large table I manage: SELECT COUNT(*) from It took 1:09 to count 10,725,948 rows in the table. At the same time, SQL Server had to perform a lot of logical and physical I/O in order to perform the count, chewing up important SQL Server resources. A much faster, and more efficient, way of counting rows in a table is to run the following query:

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SELECT rows FROM sysindexes WHERE id = OBJECT_ID('') AND indid < 2 When I run the query against the same table, it takes less than a second to run, and it gave me the same results. Not a bad improvement, and it took virtually no I/O. This is because the row count of your tables is stored in the sysindexes system table of your database. So instead of counting rows when you need to, just look up the row count in the sysindexes table. The is one potential downside to using the sysindexes table. And that this system table is not updated in real time, so it might underestimate the number of rows you actually have. Assuming you have the database option turned on to "Auto Create Statistics" and "Auto Update Statistics", the value you get should be very close to being correct, if not correct. If you can live with a very close estimate, then this is the best way to count rows in your tables. [7.0, 2000] Added 7-3-2001

20. Looking for some new tools to help performance tune your operating system? Then check out the performance tools at Sysinternals. For example, they have tools to defrag your server's swap file, among many others. And best of all, most are free. [6.5, 7.0, 2000] Added 7-3-2001

21. Do you use Enterprise Manager to access remote servers, possibly over a slow WAN link? If you do, have you ever had any problems getting Enterprise Manager to connect to the remote server? If so, the problem may lay in the fact that if Enterprise Manager cannot make a connection within 4 seconds, then the connection attempt fails. To overcome slow network connections, you can change the default Enterprise Manager timeout value from 4 seconds to any amount of time you like. To change the default timeout value, select Tools|Options from Enterprise Manager, and then select the "Connection" tab if you have SQL Server 7.0, or the "Advanced" tab if you have SQL Server 2000. Here, change the "Login time-out (seconds)" option to a higher number. [7.0, 2000] Added 7-3-2001

22. SQLDIAG.exe is a command line tools that collects information about SQL Server and writes it

[

to a text file. It can be useful for documenting or troubleshooting your SQL Server. When you run this command when SQL Server is running, the following information is collected and stored in a text file called sqldiag.txt, which is stored in the \mssql\log folder. • The text of all error logs • SQL Server registry information • SQL Server dll version information • The output from these system stored procedures: • sp_configure • sp_who • sp_lock • sp_helpdb • xp_msver • sp_helpextendedproc • sysprocesses • Input buffer SPIDs/deadlock information • Microsoft diagnostics report for the server • The last 100 queries and exceptions (if the query history trace was running) 7.0, 2000] Added 10-12-2001

23. If you are running any service pack of SQL Server 7.0 on a multiprocessor server, the number of configured processors displayed by Enterprise Manager my be incorrect. For example, while you may have two CPUs in your server, Enterprise Manager may only report one CPU. This is a known bug that has yet to be fixed in Service Packs 1 through 3 in SQL Server 7.0. The work-around to this problem is not to use the Enterprise Manager to specify the number of CPUs for SQL Server to use, but to use the sp_configure "max degree of parallelism" option instead, such as in this example: USE master GO SP_CONFIGURE 'show advanced option', 1 GO RECONFIGURE WITH OVERRIDE GO

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SP_CONFIGURE 'max degree of parallelism', 0 GO The "max degree of parallelism" option is an advanced option, so the first portion of the code above is used to turn on the "show advanced option." Once that is done, then you can set the "max degree of parallelism" option. By setting this option to "0", you are telling SQL Server to use all available CPUs in the server. See this Microsoft article for more information: http://support.microsoft.com/support/kb/articles/Q273/8/80.ASP [7.0] Added 10-23-2001

24. Memory leaks can steal valuable memory from your SQL Server, reducing performance, and perhaps even forcing you to reboot your server. A memory leak occurs when a poorly-written or buggy program requests memory from the operating system, but does not release the memory when it is done with it. Because of this, the application can use up more and more memory in a server, greatly slowing it down, and even perhaps crashing the server. Some memory leaks come from the operating system itself, device drivers, MDAC components, and even SQL Server. And of course, virtually any application can cause a memory leak, which is another good reason to dedicate a single server to SQL Server instead of sharing it among multiple applications. Memory leaks are often hard to identify, especially if they leak memory slowly. Generally, memory leaks become apparent when you notice that your server is running out of available memory and paging becomes a big problem. A symptom of this is a SQL Server that runs quickly after being rebooted, but begins to run more and more slowly as time passes, and when the system is rebooted again, it speeds up again. One way to help get rid of many memory leaks is to ensure that you always have the latest service packs or updates for your server's software. But a memory leak you find may not have an immediate fix. If this is the case, you may be forced to reboot your server periodically in order to free up memory. Identifying what is causing a memory leak is often difficult. One method involved using Performance Monitor to monitor all of the counters in the Memory object over time, seeing what is happening internally in your computer. Another method is to use Task Manager to view how much memory is used by each process. A process that seems to be using an unusual amount of memory may be the culprit. [6.5, 7.0, 2000] Added 12-11-2001

Tips on Optimizing SQL Server Indexes 1.

Indexes should be considered on all columns that are frequently accessed by the WHERE, ORDER BY, GROUP BY, TOP, and DISTINCT clauses. Without an index, each of these operations will require a table scan of your table, potentially hurting performance. Keep in mind the word "considered". An index created to support the speed of a particular query may not be the best index for another query on the same table. Sometimes you have to balance indexes to attain acceptable performance on all the various queries that are run against a table. [6.5, 7.0, 2000] Updated 12-7-2001

2.

To help identify which tables in your database may need additional or improved indexes, use the SQL Server Profiler Create Trace Wizard to run the "Identify Scans of Large Tables" trace. This trace will tell which tables are being scanned by queries instead of using an index to seek the data. This should provide you data you can use to help you identify which tables may need additional or better indexes. [7.0, 2000]

3.

Don't over index your OLTP tables, as every index you add increases the time in takes to perform INSERTS, UPDATES, and DELETES. There must be a fine line drawn between having the ideal number of indexes (for SELECTs) and the ideal number for data modifications. [6.5, 7.0, 2000]

4.

Don't automatically add indexes on a table because it seems like the right thing to do. Only add indexes if you know that they will be used by the queries run against the table. [6.5, 7.0, 2000]

5.

Don't accidentally add the same index twice on a table. This is easier to happen than you think. For example, you add a unique or primary key to an column, which of course creates an index to enforce what you want to happen. But without thinking about it when evaluating the need for indexes on a table,

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you decide to add a new index, and this new index happens to be on the same column as the unique or primary key. As long as you give indexes different names, SQL Server will allow you to create the same index over and over. [7.0, 2000] Added 4-2-2001

6.

If you have a table that is subject to many INSERTS, be sure that you have added a clustered index to it (not based on an incrementing key), whether or not it really needs one. A table that does not have a clustered index is called a heap. Every time data is INSERTed into a heap, the row is added to the end of the table. If there are many INSERTS, this spot could become a "hot spot" which could significantly affect performance. By adding a clustered index to the table (not based on an incrementing key), any potential hotspots are avoided. [6.5, 7.0, 2000] Added 2-22-2001

7.

Drop indexes that are never used by the Query Optimizer. Unused indexes slow data modifications and waste space in your database, increasing the amount of time it takes to backup and restore databases. Use the Index Wizard ( 7.0 and 2000) to help identify indexes that are not being used. [6.5, Generally, you probably won't want to add an index to a table under these conditions: • If the index is not used by the query optimizer. Use Query Analyzer's "Show Execution Plan" option to see if your queries against a particular table use an index or not. If the table is small, most likely indexes will not be used. • If the column values exhibit low selectivity, often less than 90%-95% for non-clustered indexes. • If the column(s) to be indexed are very wide. • If the column(s) are defined as TEXT, NTEXT, or IMAGE data types. • If the table is rarely queried.

8.

9.

While high index selectivity is generally an important factor that the Query Optimizer uses to determine whether or not to use an index, there is one special case where indexes with low selectivity can be useful speeding up SQL Server. This is the case for indexes on foreign keys. Whether an index on a foreign key has either high or low selectivity, an index on a foreign key can be used by the Query Optimizer to perform a merge join on the tables in question. A merge join occurs when a row from each table is taken and then they are compared to see if they match the specified join criteria. If the tables being joined have the appropriate indexes (no matter the selectivity), a merge join can be performed, which is generally much faster than a join to a table with a foreign key that does not have an index. [7.0, 2000] Added 4-9-2001

10. On data warehousing databases, which are essentially read-only, having an many indexes as necessary for covering virtually any query is not normally a problem. [6.5, 7.0, 2000]

11. To provide the up-to-date statistics the query optimizer needs to make smart query optimization decisions, you will generally want to leave the "Auto Update Statistics" database option on. This helps to ensure that the optimizer statistics are valid, helping to ensure that queries are properly optimized when they are run. But this option is not a panacea. When a SQL Server database is under very heavy load, sometimes the auto update statistics feature can update the statistics at inappropriate times, such as the busiest time of the day. If you find that the auto update statistics feature is running at inappropriate times, you may want to turn it off, and then manually update the statistics (using UPDATE STATISTICS or sp_updatestats) when the database is under a less heavy load. But again, consider what will happen if you do turn off the auto update statistics feature? While turning this feature off may reduce some stress on your server by not running at inappropriate times of the day, it could also cause some of your queries not to be properly optimized, which could also put extra stress on your server during busy times. Like many optimization issues, you will probably need to experiment to see if turning this option on or off is more effective for your environment. But as a rule of thumb, if your server is not maxed out, then leaving this option is probably the best decision. [7.0, 2000] More info from Microsoft

12. Keep the "width" of your indexes as narrow as possible, especially when creating composite (multicolumn) indexes. This reduces the size of the index and reduces the number of reads required to read the index, boosting performance. [6.5, 7.0, 2000]

13. If possible, try to create indexes on columns that have integer values instead of characters. Integer values have less overhead than character values. [6.5, 7.0, 2000]

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14. An index is generally only useful to a query if the WHERE clause of the query matches the column(s) that are leftmost in the index. So if you create a composite index, such as "City, State", then a query such as "WHERE City = 'Springfield'" will use the index, but the query "WHERE STATE = 'MO'" will not use the index. [6.5, 7.0, 2000]

15. Even if the WHERE clause in a query does not specify the first column of an available index (which normally disqualifies the index from being used), if the index is a composite index and contains all of the columns referenced in the query, the query optimizer can still use the index, because the index is a covering index. [6.5, 7.0, 2000]

16. When you create an index with a composite key, the order of the columns in the key is important. Try to order the columns in the key as to enhance selectivity, with the most selective columns to the leftmost of the key. If you don't due this, and put a non-selective column at the first part of the key, you risk having the Query Optimizer not use the index at all. Generally, a column should be at least 95% unique in order for it to be considered selective. [6.5, 7.0, 2000] Updated 3-6-2001 More info on selectivity

17. As you may know, an index is automatically created for column(s) in your table that you specify as a PRIMARY KEY or as UNIQUE. If two or more columns are involved, and a composite index is created, you should choose how the columns will be ordered in the composite index, instead of accepting the default choices offered by SQL Server. This is because you always want to use the most selective columns at the left of the key to ensure that the composite index is selective enough to be used by the Query Optimizer. If you don't do this, then the default order of the columns in the composite index may not be very selective, and the index may not be used by Query Optimizer. While the order of the columns in the index are important to the Query Optimizer, the order is not important to the PRIMARY KEY or UNIQUE constraints. [6.5, 7.0, 2000] Added 3-6-2001

18. If you have two or more tables that are frequently joined together, then the columns used for the joins should have an appropriate index. If the columns used for the joins are not naturally compact, then considering adding surrogate keys to the tables that are compact in order to reduce the size of the keys, thus decreasing I/O during the join process, increasing overall performance. [6.5, 7.0, 2000]

19. When creating indexes, try to make them unique indexes if at all possible. SQL Server can often search through a unique index faster than a non-unique index because in a unique index, each row is unique, and once it is found, SQL Server doesn't have to look any further. [6.5, 7.0, 2000] Added 10-192000

20. If a particular query against a table is run infrequently, and the addition of an index greatly speeds the performance of the query, but the performance of INSERTS, UPDATES, and DELETES is negatively affected by the addition of the index, consider creating the index for the table for the duration of when the query is run, then dropping the index. An example of this is when monthly reports are run at the end of the month on an OLTP application. [6.5, 7.0, 2000] Added 9-11-2000

21. If you like to get under the cover of SQL Server to learn more about indexing, take a look at the sysindex system table that is found in every database. Here, you can find a wealth of information on the indexes and tables in your database. To view the data in this table, run this query from the database you are interested in: SELECT * FROM sysindexes. Here are some of the more interesting fields found in this table: • dpages: If the indid value is 0 or 1, then dpages is the count of the data pages used for the index. If the indid is 255, then dpages equals zero. In all other cases, dpages is the count of the nonclustered index pages used in the index. • id: Refers to the id of the table this index belongs to. • indid: This column indicates the type of index. For example, 1 is for a clustered table, a value greater than 1 is for a non-clustered index, and a 255 indicates that the table has text or image data. • OrigFillFactor: This is the original fillfactor used when the index was first created, but it is not maintained over time. • statversion: Tracks the number of times that statistics have been updated. • status: 2 = unique index, 16 = clustered index, 64 = index allows duplicate rows, 2048 = the index is used to enforce the Primary Key constraint, 4096 = the index is used to enforce the Unique

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•

constraint. These values are additive, and the value you see in this column may be a sum of two or more of these options. used: If the indid value is 0 or 1, then used is the number of total pages used for all index and table data. If indid is 255, used is the number of pages for text or image data. In all other cases, used is the number of pages in the index.

22. Don't use FLOAT or REAL data types for primary keys, as they add unnecessary overhead and can hurt performance. [6.5, 7.0, 2000] Added 10-4-2000

23. If your WHERE clause includes an AND operator, one way to optimize it is to ensure that at least one of the search criterion is highly selective and includes an index for the relevant column. [6.5, 7.0, 2000] Added 10-17-2000

24. The Query Optimizer will always perform a table scan or a clustered index scan on a table if the WHERE clause in the query contains an OR operator and if any of the referenced columns in the OR clause are not indexed (or does not have a useful index). Because of this, if you use many queries with OR clauses, you will want to ensure that each referenced column has an index. [7.0, 2000] Added 1017-2000

25. The Query Optimizer converts the Transact-SQL IN clause to the OR operator when parsing your code. Because of this, keep in mind that if the referenced column in your query doesn't include an index, then the Query Optimizer will perform a table scan or clustered index scan on the table. [6.5, 7.0, 2000] Added 10-17-2000

26. If you use the SOUNDEX function against a table column in a WHERE clause, the Query Optimizer will ignore any available indexes and perform a table scan. If your table is large, this can present a major performance problem. If you need to perform SOUNDEX type searches, one way around this problem is to precalculate the SOUNDEX code for the column you are searching and then place this value in a column of its own, and then place an index on this column in order to speed searches. [6.5, 7.0, 2000] Added 11-9-2000

27. If you need to create indexes on large tables, you may be able to speed up their creation by using the new SORT_IN_TEMPDB option available with the CREATE INDEX command. This option tells SQL Server to use the tempdb database, instead of the current database, to sort data while creating indexes. Assuming your tempdb database is isolated on its own separate disk or disk array, then the process of creating the index can be sped up. The only slight downside to using this option is that it takes up slightly more disk space than if you didn't use it, but this shouldn't be much of an issue in most cases. If your tempdb database is not on its own disk or disk array, then don't use this option, as it can actually slow performance. [2000] Added 10-19-2000

28. SQL Server 2000 Enterprise Edition (not the standard edition) offers the ability to create indexes in parallel, greatly speeding index creation. Assuming your server has multiple CPUs, SQL Server 2000 uses near-linear scaling to boost index creation speed. For example, using two CPUs instead of one CPU almost halves the speed it takes to create indexes. [2000] Added 12-19-2000 29. As you probably already know, indexes on narrow columns are preferable to indexes on wide columns. The narrower the index, the more entries SQL Server can fit on a data page, which in turn reduces the amount of I/O required to access the data. But sometimes the column you want to search on using an index is much wider than desirable. For example, let's say you have a music database that lists the titles of over 5,000,000 songs, and that you want to search by song title. Also assume that the column used to store the music titles is a VARCHAR(45). Forty-five characters is a very wide index, and creating an index on such a wide column is not wise from a performance perspective. So how do we deal with such a scenario? SQL Server 2000 offers a new function called CHECKSUM. The main purpose for this function is to create what are called hash indices. A hash indices is an index built on a column that stores the checksum of the data found in another column in the table. The CHECKSUM function takes data from another column and creates a checksum value. In other words, the CHECKSUM function is used to create a mostly unique value that represents other data in your table. In most cases, the CHECKSUM value will be much smaller than the actual value. For the most part, checksum values are unique, but this is not guaranteed. It is possible that two slightly different values may produce the same identical CHECKSUM value.

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Here's how this works using our music database example. Say we have a song with the title "My Best Friend is a Mule from Missouri". As you can see, this is a rather long value, and adding an index to the song title column would make for a very wide index. But in this same table, we can add a CHECKSUM column that takes the title of the song and creates a checksum based on it. In this case, the checksum would be 1866876339. The CHECKSUM function always works the same, so if you perform the CHECKSUM function on the same value many different times, you would always get the same result. So how does the CHECKSUM help us? The advantage of the CHECKSUM function is that instead of creating a wide index by using the song title column, we create an index on the CHECKSUM column instead. "That's fine and dandy, but I thought you wanted to search by the song's title? How can anybody ever hope to remember a checksum value in order to perform a search?" Here's how. Take a moment to review this code: SELECT title, artist, composer FROM songs WHERE title = 'My Best Friend is a Mule from Missouri' AND checksum_title = CHECKSUM('My Best Friend is a Mule from Missouri') In this example, it appears that we are asking the same question twice, and in a sense, we are. The reason we have to do this is because there may be checksum values that are identical, even though the names of the songs are different. Remember, unique checksum values are not guaranteed. Here's how the query works. When the Query Optimizer examines the WHERE clause, it determines that there is an index on the checksum_title column. And because the checksum_title column is highly selective (minimal duplicate values) the Query Optimizer decides to use the index. In addition, the Query Optimizer is able to perform the CHECKSUM function, converting the song's title into a checksum value and using it to locate the matching records in the index. Because an index is used, SQL Server can very quickly locate the rows that match the second part of the WHERE clause. Once the rows have been narrowed down by the index, then all that has to be done is to compare these matching rows to the first part of the WHERE clause, which will take very little time. This may seem a lot of work to shorten the width of an index, but in many cases, this extra work will pay off in better performance in the long run. Because of the nature of this tip, I suggest you experiment using this method, and the more conventional method of creating an index on the title column itself. Since there are so many variables to consider, it is tough to know which method is better in your particular situation unless you give them both a try. [2000] Added 3-6-2001

30. There is a bug in SQL Server 7.0 and 2000 that has yet to be corrected, that can negatively affect the performance of some queries. Queries that have multiple OR clauses in them that are based on a clustered composite index may ignore the index and perform a table scan instead. This bug only appears if the query is within a stored procedure, or if it is executed through an ODBC-based application, such as VB, ASP, or Microsoft Access. The best way to identify if you are experiencing this bug is to view the Query Plan for slow queries that fit the criteria above, and see if a table scan is being performed, or if the index is being used. There are five different possible workarounds for this bug, depending on your circumstances: • Use an appropriate index hint to force the use of the composite index. This is probably the easiest method to get around this problem, and the one I recommend. • Changing from a clustered composite index to a non-clustered composite index may help, but this is not guaranteed. You will have to test it for yourself. • Rewrite the query using a UNION clause to combine the results returned from the OR clauses. Of course, using a UNION clause may itself degrade performance. You will have to test this option to see if it is faster or not. • If the query is being executed from an ODBC application through the SQLPrepare function with the SQL Server ODBC driver version 3.6 or earlier, then you can disable the "Generate Stored Procedures for Prepared Statements" option to workaround the bug. • If the query is being executed from an ODBC application through either the SQLPrepare or SQLExecDirect functions with a parameterized query using the SQL Server ODBC driver version 3.7, you can use the odbccmpt utility to enable the SQL Server 6.5 ODBC compatibility option and also disable the "Generate Stored Procedures for Prepared Statements" option to workaround the bug.

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SQL Server Performance Tuning Tips for Stored Procedures 1.

Whenever a client application needs to send Transact-SQL to SQL Server, send it in the form of a stored procedure instead of a script or embedded Transact-SQL. This not only reduces network traffic (only the EXECUTE or CALL is issued over the network), but it can speed up the Transact-SQL because the code in the stored procedure residing on the server is already pre-compiled. In addition, after a stored procedure is run for the first time, it stays cached in SQL Server’s memory where it can potentially be reused, further reducing overhead on the SQL Server. Keep in mind that just because you use a stored procedure does not mean that it will run fast. The code you use within your stored procedure must be well designed for both speed and reuse. [6.5, 7.0, 2000] Updated 5-18-2001

2.

To help identify performance problems with stored procedures, use the SQL Server's Profiler Create Trace Wizard to run the "Profile the Performance of a Stored Procedure" trace to provide you with the data you need to identify poorly performing stored procedures. [7.0, 2000]

3.

Include in your stored procedures the statement, "SET NOCOUNT ON". If you don't turn this command on, then every time a SQL statement is executed, SQL Server will send a response to the client indicating the number of rows affected by the statement. It is rare that this information will ever be needed by the client. Using this statement will help reduce the traffic between the server and the client. [6.5, 7.0, 2000]

4.

Keep Transact-SQL transactions as short as possible. This helps to reduce the number of locks, helping to speed up the overall performance of your SQL Server application. Two ways to help reduce the length of a transaction are to: 1) break up the entire job into smaller steps so each step can be committed as soon as possible; and 2) take advantage of SQL Server statement batches, which acts to reduce the number of round-trips between the client and server. [6.5, 7.0, 2000]

5.

When a stored procedure is first executed (and it does not have the WITH RECOMPILE option), it is optimized and a query plan is compiled and cached in SQL Server's buffer. If the same stored procedure is called again from the same connection, it will used the cached query plan instead of creating a new one, saving time and boosting performance. This may or may not be what you want. If the query in the stored procedure is exactly the same each time, then this is a good thing. But if the query is dynamic (the WHERE clauses changes from one execution of the stored procedure to the next), then this is a bad thing, as the query will not be optimized when it is run, and the performance of the query can suffer greatly. If you know that your query will vary each time it is run from the stored procedure, you will want to add the WITH RECOMPILE option when you create the stored procedure. This will force the stored procedure to be re-compiled each time it is run, ensuring the query is optimized each time it is run. [6.5, 7.0, 2000]

6.

Design your application to allow your users to cancel running queries. Not doing so may force the user to reboot the client, which can cause unresolvable performance problems. [6.5, 7.0, 2000]

7.

Many stored procedures have the option to accept multiple parameters. This in and of itself is not a bad thing. But what can often cause problems is if the parameters are optional, and the number of parameters varies greatly each time the stored procedure runs. There are two ways to handle this problem, the slow performance way and fast performance way. If you want to save your development time, but don't care about your application's performance, you can write your stored procedure generically so that it doesn't care how many parameters it gets. The problem with this method is that you may end up unnecessarily joining tables that don't need to be joined based on the parameters submitted for any single execution of the stored procedure. Another, much better performing way, although it will take you more time to code, is to include IF...ELSE logic in your stored procedure, and create separate queries for each possible combination of parameters that are to be submitted to the stored procedure. This way, you can be sure you query is as efficient as possible each time it runs. [6.5, 7.0, 2000] Added 12-29-2000

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8.

Here's another way to handle the problem of not knowing what parameters your stored procedure might face. In fact, it will probably perform faster than the tip listed above. Although the above tip is a good starting point, it's not complete. The problem is the query-plans, the pre-compilation of stored procedures, that SQL Server does for you. As you know, one of the biggest reasons to use stored procedures instead of ad-hoc queries is the performance gained by using them. The problem that arises with the above tip is that SQL Server will only generate a query-plan for the path taken through your stored procedure when you first call it, not all possible paths. Let me illustrate this with an example. Consider the following procedure (pre-compilation doesn't really have a huge effect on the queries used here, but these are just for illustration purposes): CREATE PROCEDURE dbo.spTest (@query bit) AS IF @query = 0 SELECT * FROM authors ELSE SELECT * FROM publishers GO Suppose I make my first call to this procedure with the @query parameter set to 0. The query-plan that SQL Server will generate will be optimized for the first query ("SELECT * FROM authors"), because the path followed on the first call will result in that query being executed. Now, if I next call the stored procedure with @query set to 1, the query plan that SQL Server has in memory will not be of any use in executing the second query, since the query-plan is optimized for the authors table, not the publishers table. Result: SQL Server will have to compile a new query plan, the one needed for the second query. If I next call the procedure with @query set to 0 again, the whole path will have to be followed from the start again, since only one query-plan will be kept in memory for each stored procedure. This will result in sub-optimal performance. As it happens I have a solution, one that I've used a lot with success. It involves the creation of what I like to call a 'delegator'. Consider again spTest. I propose to rewrite it like this: CREATE PROCEDURE dbo.spTestDelegator (@query bit) AS IF @query = 0 EXEC spTestFromAuthors ELSE EXEC spTestFromPublishers GO CREATE PROCEDURE dbo.spTestFromAuthors AS SELECT * FROM authors GO CREATE PROCEDURE dbo.spTestFromPublishers AS SELECT * FROM publishers GO The result of this restructuring will be that there will always be an optimized query-plan for spTestFromAuthors and spTestFromPublishers, since they only hold one query. The only one getting re-compiled over and over again is the delegator, but since this stored procedure doesn't actually hold any queries, that won't have a noticeable effect on execution time. Of course re-compiling a plan for a simple 'SELECT *' from a single table will not give you a noticeable delay either (in fact, the overhead of an extra stored procedure call may be bigger then the re-compilation of "SELECT * FROM AnyTable"), but as soon as the queries get bigger, this method certainly pays off. The only downside to this method is that now you have to manage three stored procedures instead of one. This is not that much of a problem though as the different stored procedures can be considered one single 'system', so it would be logical to keep all of them together in the same script, which would be just as easy to edit as a single stored procedure would be. As far as security is concerned, this method shouldn't give you any extra headaches either, as the delegator is the only stored procedure directly called by the client, this is the only one you need to manage permissions on. The rest will only be called by the delegator, which will always work as long as those stored procedures are owned by the

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same user as the delegator. I've had large successes using this technique. Recently I developed a (partial full-text) search engine for our reports database, which resulted in a stored procedure that originally ran about 20 seconds. After employing the above technique, the stored procedure only took about 2 seconds to run, resulting in a ten-fold increase in performance! [6.5, 7.0, 2000] Contributed by Jeremy van Dijk. Added 8-15-2000

9.

While temporary stored procedures can provide a small performance boost in some circumstances, using a lot of temporary stored procedures in your application can actually create contention in the system tables and hurt performance. Instead of using temporary stored procedures, you may want to consider using the SP_EXECUTESQL stored procedure instead. It provides the same benefits on temporary stored procedures, but it does not store data in the systems tables, avoiding the contention problems. [7.0, 2000]

10. If you are creating a stored procedure to run in a database other than the Master database, don't use the prefix "sp_" in its name. This special prefix is reserved for system stored procedures. Although using this prefix will not prevent a user defined stored procedure from working, what it can do is to slow down its execution ever so slightly. The reason for this is that by default, any stored procedure executed by SQL Server that begins with the prefix "sp_", is first attempted to be resolved in the Master database. Since it is not there, time is wasted looking for the stored procedure. If SQL Server cannot find the stored procedure in the Master database, then it next tries to resolve the stored procedure name as if the owner of the object is "dbo". Assuming the stored procedure is in the current database, it will then execute. To avoid this unnecessary delay, don't name any of your stored procedures with the prefix "sp_". [6.5, 7.0, 2000] Tip contributed by Joey Allen.

11. Before you are done with your stored procedure code, review it for any unused code that you may have forgotten to remove while you were making changes, and remove it. Unused code just adds unnecessary bloat to your stored procedures. [6.5, 7.0, 2000] Added 8-15-2000

12. For best performance, all objects that are called within the same stored procedure should all be owned by the same owner, preferably dbo. If they are not, then SQL Server must perform name resolution on the objects if the object names are the same but the owners are different. When this happens, SQL Server cannot use a stored procedure "in-memory plan" over, instead, it must re-execute the stored procedure, which hinders performance. [7.0, 2000] Added 10-12-2000

13. When you need to execute a string of Transact-SQL, you should use the sp_executesql stored procedure instead of the EXECUTE statement. Sp_executesql offers to major advantages over EXECUTE. First, it supports parameter substitution, which gives your more options when creating your code. Second, it creates query execution plans that are more likely to be reused by SQL Server, which in turn reduces overhead on the server, boosting performance. Sp_executesql executes a string of Transact-SQL in its own self-contained batch. When it is run, SQL Server compiles the code in the string into an execution plan that is separate from the batch that contained the sp_executesql and its string. Learn more about how to use sp_executesql in the SQL Server Books Online. [7.0, 2000] Added 3-72001

14. SQL Server will automatically recompile a stored procedure if any of the following happens: • • • • • • • • • •

If you include a WITH RECOMPILE clause in a CREATE PROCEDURE or EXECUTE statement. If you run sp_recompile for any table referenced by the stored procedure. If any schema changes occur to any of the objects referenced in the stored procedure. This includes adding or dropping rules, defaults, and constraints. New distribution statistics are generated. If you restore a database that includes the stored procedure or any of the objects it references. If the stored procedure is aged out of SQL Server's cache. An index used by the execution plan of the stored procedure is dropped. A major number of INSERTS, UPDATES or DELETES are made to a table referenced by a stored procedure. The stored procedure includes both DDL (Data Definition Language) and DML (Data Manipulation Language) statements, and they are interleaved with each other. If the stored procedure performs certain actions on temporary tables.

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15. One hidden performance problem of using stored procedures is when a stored procedure recompiles too often. Normally, you want a stored procedure to compile once and to be stored in SQL Server's cache so that it can be re-used without it having to recompile each time it is used. This is one of the major benefits of using stored procedures. But in some cases, a stored procedure is recompiled much more often than it needs to be recompiled, hurting your server's performance. In fact, it is possible for a stored procedure to have to be recompiled while it is executing! Here are three potential problems you want to look out for when writing stored procedures. Unnecessary Stored Procedure Recompilations Due to Row Modifications and Automated Statistics Update If your database has the "Auto Update Statistics" database option turned on, SQL Server will periodically automatically update the index statistics. On a busy database, this could happen many times each hour. Normally, this is a good thing because the Query Optimizer needs current index statistics if it is to make good query plan decisions. One side effect of this is that this also causes any stored procedures that reference these tables to be recompiled. Again, this is normal, as you don't want a stored procedure to be running an outdated query plan. But again, sometimes stored procedures recompile more than they have to. Here are some suggestions on how to reduce some of the unnecessary recompilations: • Use sp_executesql instead of EXECUTE to run Transact-SQL strings in your stored procedures. • Instead of writing one very large stored procedure, instead break down the stored procedure into two or more sub-procedures, and call then from a controlling stored procedure. • If your stored procedure is using temporary tables, use the KEEP PLAN query hint, which is used to stop stored procedure recompilations caused by more than six changes in a temporary table, which is the normal behavior. This hint should only be used for stored procedures than access temporary tables a lot, but don't make many changes to them. If many changes are made, then don't use this hint. Unnecessary Stored Procedure Recompilations Due to Mixing DDL and DML Statements in the Same Stored Procedure If you have a DDL (Data Definition Language) statement in your stored procedure, the stored procedure will automatically recompile when it runs across a DML (Data Manipulation Language) statement for the first time. And if you intermix both DDL and DML many times in your stored procedure, this will force a recompilation every time it happens, hurting performance. To prevent unnecessary stored procedure recompilations, you should include all of your DDL statements at the first of the stored procedure so they are not intermingled with DML statements. Unnecessary Stored Procedure Recompilations Due to Specific Temporary Table Operations Improper use of temporary tables in a stored procedure can force them to be recompiled every time the stored procedure is run. Here's how to prevent this from happening: • Any references to temporary tables in your stored procedure should only refer to tables created by that stored procedure, not to temporary tables created outside your stored procedure, or in a string executed using either the sp_executesql or the EXECUTE statement. • All of the statements in your stored procedure that include the name of a temporary table should appear syntactically after the temporary table. • The stored procedure should not declare any cursors that refer to a temporary table. • Any statements in a stored procedure that refer to a temporary table should precede any DROP TABLE statement found in the stored procedure. • The stored procedure should not create temporary tables inside a control-of-flow statement.

16. To find out if your SQL Server is experiencing excessive recompilations of stored procedures, a common cause of poor performance, create a trace using Profiler and track the SP:Recompile event. A large number of recompilations should be an indicator if you potentially have a problem. Identify which stored procedures are causing the problem, and then take correction action (if possible) to reduce or eliminate these excessive recompilations. [7.0, 2000] Added 9-13-2001

17. Stored procedures can better boost performance if they are called via Microsoft Transaction Server (MTS) instead of being called directly from your application. A stored procedure can be reused from the procedure cache only if the connection settings calling the stored procedure are the same. If different connections call a stored procedure, SQL Server must load a separate copy of the stored procedure for each connection, which somewhat defeats the purpose of stored procedures. But if the same connection calls a stored procedure, it can be used over and over from the procedure cache. The advantage of Transaction Server is that it reuses connections, which means that stored procedures can

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be reused more often. If you write an application where every user opens their own connection, then stored procedures cannot be reused as often, reducing performance. [7.0, 2000] Added 10-12-2000

18. Avoid nesting stored procedures, although it is perfectly legal to do so. Nesting not only makes debugging more difficult, it makes it much more difficult to identify and resolve performance-related problems. [6.5, 7.0, 2000] Added 10-12-2000

19. If you use input parameters in your stored procedures, you should validate all of them at the beginning of your stored procedure. This way, if there is a validation problem and the client applications needs to be notified of the problem, it happens before any stored procedure processing takes place, preventing wasted effort and boosting performance. [6.5, 7.0, 2000] Added 10-12-2000

20. When calling a stored procedure from your application, it is important that you call it using its fully qualified name. Such as: exec database_name.dbo.myProcedure instead of: exec myProcedure Why? There are a couple of reasons, one of which relates to performance. First, using fully qualified names helps to eliminate any potential confusion about which stored procedure you want to run, helping to prevent bugs and other potential problems. But more importantly, doing so allows SQL Server to access the stored procedures execution plan more directly, and in turn, speeding up the performance of the stored procedure. Yes, the performance boost is very small, but if your server is running tens of thousands or more stored procedures every hour, these little time savings can add up. [7.0, 2000] Added 3-7-2001 More from Microsoft

21. If a stored procedure needs to return only a single value, and not a recordset, consider returning the single value as an output statement. While output statements are generally used for errorchecking, they can actually be used for any reason you like. Returning a single value as at output statement is faster than returning a single value as part of a recordset. [6.5, 7.0, 2000] Added 8-1-2001

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