Goals

Oracle SQL tuning goals Oracle SQL tuning is a phenomenally complex subject. Entire books have been written about the nuances of Oracle SQL tuning; however, there are some general guidelines that every Oracle DBA follows in order to improve the performance of their systems. Again, see the book "Oracle Tuning: The Definitive Reference", for complete details. The goals of SQL tuning focus on improving the execution plan to fetch the rows with the smallest number of database "touches" (LIO buffer gets and PIO physical reads). •

Remove unnecessary large-table full-table scans—Unnecessary full-table scans cause a huge amount of unnecessary I/O and can drag-down an entire database. The tuning expert first evaluates the SQL based on the number of rows returned by the query. If the query returns less than 40 percent of the table rows, it needs tuning. The most common tuning remedy for unnecessary fulltable scans is adding indexes. Standard b-tree indexes can be added to tables, and bitmapped and function-based indexes can also eliminate full-table scans. In some cases, an unnecessary fulltable scan can be forced to use an index by adding an index hint to the SQL statement.

•

Cache small-table full-table scans—In cases where a full-table scan is the fastest access method, the administrator should ensure that a dedicated data buffer is available for the rows. In Oracle7, you can issue "alter table xxx cache". In Oracle8 and beyond, the small table can be cached by forcing it into the KEEP pool.

•

Verify optimal index usage—This is especially important when using the rule-based optimizer. Oracle sometimes has a choice of indexes, and the tuning professional must examine each index and ensure that Oracle is using the proper index.

•

Materialize your aggregations and summaries for static tables - One features of the Oracle 10g SQLAccess advisor is recommendations for new indexes and suggestions for materialized views. Materialized views pre-join tables and pre-summarize data, a real silver bullet for data mart reporting databases where the data is only updated daily. Again, see the book "Oracle Tuning: The Definitive Reference", for complete details on SQL tuning with materialized views.

These are the goals of SQL tuning in a nutshell. However, they are deceptively simple, and to effectively meet them, we need to have a through understanding of the internals of Oracle SQL. Let's begin with an overview of the Oracle SQL optimizers. Oracle SQL optimizers One of the first things the Oracle DBA looks at is the default optimizer mode for the database. The Oracle initialization parameters offer many cost-based optimizer modes as well as the deprecated yet useful rulebased hint: The cost-based optimizer uses “statistics” that are collected from the table using the “analyze table” command. Oracle uses these metrics about the tables in order to intelligently determine the most efficient way of servicing the SQL query. It is important to recognize that in many cases, the cost-based optimizer may not make the proper decision in terms of the speed of the query. The cost-based optimizer is constantly being improved, but there are still many cases in which the rule-based optimizer will result in faster Oracle queries. A strategic plan for Oracle SQL tuning Many people ask where they should start when tuning Oracle SQL. Tuning Oracle SQL is like fishing. You must first fish in the Oracle library cache to extract SQL statements and rank the statements by their

Tuning SQL statements again...

1

amount of activity. Step 1—Identify high-impact SQL The SQL statements will be ranked according the number of executions and will be tuned in this order. The executions column of the v$sqlarea view and the stats$sql_summary or the dba_hist_sql_summary table can be used to locate the most frequently used SQL. Note that we can display SQL statements by: •

Rows processed—Queries that process a large number of rows will have high I/O and may also have impact on the TEMP tablespace.

•

Buffer gets—High buffer gets may indicate a resource-intensive query.

•

Disk reads—High disk reads indicate a query that is causing excessive I/O.

•

Memory KB—The memory allocation of a SQL statement is useful for identifying statements that are doing in-memory table joins.

•

CPU secs—This identifies the SQL statements that use the most processor resources.

•

Sorts—Sorts can be a huge slowdown, especially if they’re being done on a disk in the TEMP tablespace.

•

Executions—The more frequently executed SQL statements should be tuned first, since they will have the greatest impact on overall performance.

Step 2—Determine the execution plan for SQL As each SQL statement is identified, it will be “explained” to determine its existing execution plan. There are a host of third-party tools on the market that show the execution plan for SQL statements. The most common way of determining the execution plan for a SQL statement is to use Oracle's explain plan utility. By using explain plan, the Oracle DBA can ask Oracle to parse the statement and display the execution class path without actually executing the SQL statement. To see the output of an explain plan, you must first create a “plan table.” Oracle provides a script in $ORACLE_HOME/rdbms/admin called utlxplan.sql. Execute utlxplan.sql and create a public synonym for the plan_table: sqlplus > @utlxplan Table created. sqlplus > create public synonym plan_table for sys.plan_table; Synonym created.

Most relational databases use an explain utility that takes the SQL statement as input, runs the SQL optimizer, and outputs the access path information into a plan_table, which can then be interrogated to see the access methods. Listing 1 runs a complex query against a database.

EXPLAIN PLAN SET STATEMENT_ID = 'test1' FOR SET STATEMENT_ID = 'RUN1' INTO plan_table FOR SELECT 'T'||plansnet.terr_code, 'P'||detplan.pac1 || detplan.pac2 || detplan.pac3, 'P1', sum(plansnet.ytd_d_ly_tm), sum(plansnet.ytd_d_ty_tm),

Tuning SQL statements again...

2

sum(plansnet.jan_d_ly), sum(plansnet.jan_d_ty), FROM plansnet, detplan WHERE plansnet.mgc = detplan.mktgpm AND detplan.pac1 in ('N33','192','195','201','BAI', 'P51','Q27','180','181','183','184','186','188', '198','204','207','209','211') GROUP BY 'T'||plansnet.terr_code, 'P'||detplan.pac1 || detplan.pac2 || detplan.pac3;

This syntax is piped into the SQL optimizer, which will analyze the query and store the plan information in a row in the plan table identified by RUN1. Please note that the query will not execute; it will only create the internal access information in the plan table. The plan tables contains the following fields: •

operation—The type of access being performed. Usually table access, table merge, sort, or index operation

•

options—Modifiers to the operation, specifying a full table, a range table, or a join

•

object_name—The name of the table being used by the query component

•

Process ID—The identifier for the query component

•

Parent_ID—The parent of the query component. Note that several query components may have the same parent.

Now that the plan_table has been created and populated, you may interrogate it to see your output by running the following query in Listing 2. plan.sql - displays contents of the explain plan table SET PAGES 9999; SELECT lpad(' ',2*(level-1))||operation operation, options, object_name, position FROM plan_table START WITH id=0 AND statement_id = 'RUN1' CONNECT BY prior id = parent_id AND statement_id = 'RUN1';

Listing 3 shows the output from the plan table shown in Listing 1. This is the execution plan for the statement and shows the steps and the order in which they will be executed. SQL> @list_explain_plan OPERATION ------------------------------------------------------------------------------------OPTIONS OBJECT_NAME POSITION ------------------------------ ------------------------------------------------------SELECT STATEMENT SORT GROUP BY 1 CONCATENATION 1 NESTED LOOPS 1 TABLE ACCESS FULL PLANSNET 1 TABLE ACCESS BY ROWID DETPLAN 2 INDEX RANGE SCAN DETPLAN_INDEX5 1 NESTED LOOPS

From this output, we can see the dreaded TABLE ACCESS FULL on the PLANSNET table. To diagnose

Tuning SQL statements again...

3

the reason for this full-table scan, we return to the SQL and look for any plansnet columns in the WHERE clause. There, we see that the plansnet column called “mgc” is being used as a join column in the query, indicating that an index is necessary on plansnet.mgc to alleviate the full-table scan. While the plan table is useful for determining the access path to the data, it does not tell the entire story. The configuration of the data is also a consideration. The SQL optimizer is aware of the number of rows in each table (the cardinality) and the presence of indexes on fields, but it is not aware of data distribution factors such as the number of expected rows returned from each query component.

Step 3—Tune the SQL statement For those SQL statements that possess a nonoptimal execution plan, the SQL will be tuned by one of the following methods:

•

Adding SQL “hints” to modify the execution plan

• •

Re-write SQL with Global Temporary Tables Rewriting the SQL in PL/SQL. For certain queries this can result in more than a 20x performance improvement. The SQL would be replaced with a call to a PL/SQL package that contained a stored procedure to perform the query.

Using hints to tune Oracle SQL Among the most common tools for tuning SQL statements are hints. A hint is a directive that is added to the SQL statement to modify the access path for a SQL query. Oracle publishes many dozens of SQL hints, and hints become increasingly more complicated through the various releases of Oracle and on into Oracle. Note: Hints are only used for de-bugging and you should adjust your optimizer statistics to make the CBO replicate the hinted SQL. Let’s look at the most common hints to improve tuning: • •

Mode hints: first_rows_10, first_rows_100 Oracle leading and ordered hints Also see how to tune table join order with histograms

•

Dynamic sampling: dynamic_sampling

• •

Oracle SQL undocumented tuning hints - Guru's only The cardinality hint

Self-order the table joins - If you find that Oracle is joining the tables together in a sub-optimal order, you can use the ORDERED hint to force the tables to be joined in the order that they appear in the FROM clause. See

Try a first_rows hint. Oracle has two cost-based optimizer modes, first_rows and all_rows. The first_rows mode will execute to begin returning rows as soon as possible, whereas the all_rows mode is designed to optimize the resources on the entire query before returning rows.

Tuning SQL statements again...

4

SELECT /*+ first_rows */

A case study in SQL tuning One of the historic problems with SQL involves formulating SQL queries. Simple queries can be written in many different ways, each variant of the query producing the same result—but with widely different access methods and query speeds. For example, a simple query such as “What students received an A last semester?” can be written in three ways, as shown in below, each returning an identical result.

A standard join: SELECT * FROM STUDENT, REGISTRATION WHERE STUDENT.student_id = REGISTRATION.student_id AND REGISTRATION.grade = 'A'; A nested query: SELECT * FROM STUDENT WHERE student_id = (SELECT student_id FROM REGISTRATION WHERE grade = 'A' ); A correlated subquery: SELECT * FROM STUDENT WHERE 0 < (SELECT count(*) FROM REGISTRATION WHERE grade = 'A' AND student_id = STUDENT.student_id );

Let’s wind up with a review of the basic components of a SQL query and see how to optimize a query for remote execution. Tips for writing more efficient SQL Space doesn’t permit me to discuss every detail of Oracle tuning, but I can share some general rules for writing efficient SQL in Oracle regardless of the optimizer that is chosen. These rules may seem simplistic but following them in a diligent manner will generally relieve more than half of the SQL tuning problems that are experienced:

•

Rewrite complex subqueries with temporary tables - Oracle created the global temporary table (GTT) and the SQL WITH operator to help divide-and-conquer complex SQL sub-queries (especially those with with WHERE clause subqueries, SELECT clause scalar subqueries and

Tuning SQL statements again...

5

FROM clause in-line views). Tuning SQL with temporary tables (and materializations in the WITH clause) can result in amazing performance improvements. •

Use minus instead of EXISTS subqueries - Some say that using the minus operator instead of NOT IN and NOT Exists will result in a faster execution plan.

•

Use SQL analytic functions - The Oracle analytic functions can do multiple aggregations (e.g. rollup by cube) with a single pass through the tables, making them very fast for reporting SQL.

•

Re-write NOT EXISTS and NOT EXISTS subqueries as outer joins - In many cases of NOT queries (but ONLY where a column is defined as NULL), you can re-write the uncorrelated subqueries into outer joins with IS NULL tests. Note that this is a non-correlated sub-query, but it could be re-written as an outer join.

select book_key from book where book_key NOT IN (select book_key from sales);

Below we combine the outer join with a NULL test in the WHERE clause without using a sub-query, giving a faster execution plan. select b.book_key from book b, sales s where b.book_key = s.book_key(+) and s.book_key IS NULL;

•

Index your NULL values - If you have SQL that frequently tests for NULL, consider creating an index on NULL values. To get around the optimization of SQL queries that choose NULL column values (i.e. where emp_name IS NULL), we can create a function-based index using the null value built-in SQL function to index only on the NULL columns.

•

Leave column names alone - Never do a calculation on an indexed column unless you have a matching function-based index (a.k.a. FBI). Better yet, re-design the schema so that common where clause predicates do not need transformation with a BIF:

where salary*5 > :myvalue where substr(ssn,7,4) = "1234" where to_char(mydate,mon) = "january"

•

Avoid the use of NOT IN or HAVING. Instead, a NOT EXISTS subquery may run faster (when appropriate).

•

Avoid the LIKE predicate = Always replace a "like" with an equality, when appropriate.

•

Never mix data types - If a WHERE clause column predicate is numeric, do not to use quotes. For char index columns, always use quotes. There are mixed data type predicates:

where cust_nbr = "123" where substr(ssn,7,4) = 1234

•

Use decode and case - Performing complex aggregations with the “decode” or "case" functions can minimize the number of times a table has to be selected.

•

Don't fear full-table scans - Not all OLTP queries are optimal when they uses indexes. If your query will return a large percentage of the table rows, a full-table scan may be faster than an index

Tuning SQL statements again...

6

scan. This depends on many factors, including your configuration (values for db_file_multiblock_read_count, db_block_size), query parallelism and the number of table/index blocks in the buffer cache. •

Use those aliases - Always use table aliases when referencing columns.

Recognizing Bottlenecks Effective operation of the Oracle database depends on an efficient and unconstricted flow of SQL and/or data among user processes, Oracle processes, Oracle shared memory, and disk structures; Figure 2 illustrates some of these process flows. To understand process flows within an Oracle instance, consider this short SQL transaction, which is illustrated in Figure 2: select * from employees where employee_id=:1 for update of salary; update employees set salary=:2 where employee_id=:1; commit;

The numbered labels in Figure 2 correspond to these activities: 1. The client program (SQL*Plus, Oracle Power Objects, or some other tool) sends the SELECT statement to the server process.

Tuning SQL statements again...

7

2. The server process looks in the shared pool for a matching SQL statement. If none is found, the server process will parse the SQL and insert the SQL statement into the shared pool. Parsing the SQL statement requires CPU and inserting a new statement into the shared pool requires a latch, an Oracle internal lock that prevents processes from concurrently updating the same area within the SGA. 3. The server process looks in the buffer cache for the data blocks required. If found, the data block must be moved on to the most-recently used end of the Least Recently Used (LRU) list. This too requires a latch. 4. If the block cannot be found in the buffer cache, the server process must fetch it from the disk file, which will require a disk I/O. A latch must be acquired before the new block can be moved into the buffer cache. 5. The server process returns the rows retrieved to the client process, which may involve a network or communications delay. 6. When the client issues the UPDATE statement, the process of parsing the SQL and retrieving the rows to be updated must occur. The update statement then changes the relevant blocks in shared memory and also entries in the rollback segment buffers. 7. The update statement will also make an entry in the redo log buffer that records the transaction details. 8. The database writer background process copies modified blocks from the buffer cache to the database files. The Oracle session performing the update needn't wait for this to occur. 9. When the COMMIT statement is issued, the logwriter process must copy the contents of the redo log buffer to the redo log file. The COMMIT statement will not return control to the Oracle session issuing the commit until this write is complete. 10. If running in ARCHIVELOG mode, the archiver process will copy full redo logs to the archive destination. A redo log will not be eligible for reuse until it has been archived. 11. At regular intervals, or when a redo log switch occurs, Oracle performs a checkpoint. A checkpoint requires that all modified blocks in the buffer cache be written to disk. A redo log file cannot be reused until the checkpoint completes. The goal in tuning and monitoring the Oracle instance is to ensure that data and instructions flow smoothly through and among the various processes and that none of these flows becomes a bottleneck for the system as a whole. Monitoring scripts and tools can be used to detect any blockages or inefficiencies in each of the processing steps previously outlined.

Tuning SQL statements again...

8

Tuning SQL statements again...

9