Oracle Sql Query Tuning Hints

Oracle SQL Query Tuning Hints WHERE Clause Try to avoid operations on database objects referenced in the WHERE clause. Given Query

Alternative

SELECT ename, hiredate, sal FROM emp WHERE SUBSTR(ename,1,3) = 'SCO';

SELECT ename, hiredate, sal FROM emp WHERE ename LIKE 'SCO%';

VARIABLE name VARCHAR2(20) exec name := 'SCOTT'

VARIABLE name VARCHAR2(20) exec name := 'SCOTT'

SELECT ename, hiredate, sal FROM emp WHERE ename = NVL (:name, ename);

SELECT ename, hiredate, sal FROM emp WHERE ename LIKE NVL (:name, '%');

SELECT ename, hiredate, sal FROM emp WHERE TRUNC (hiredate) = TRUNC (SYSDATE);

SELECT ename, hiredate, sal FROM emp WHERE hiredate BETWEEN TRUNC (SYSDATE) AND TRUNC (SYSDATE) + .99999;

SELECT ename, hiredate, sal FROM emp WHERE ename || empno = 'SCOTT7788';

SELECT ename, hiredate, sal FROM emp WHERE ename = 'SCOTT AND empno = 7788;

SELECT ename, hiredate, sal FROM emp WHERE sal + 3000 < 5000;

SELECT ename, hiredate, sal FROM emp WHERE sal < 2000;

SELECT ename, hiredate, sal FROM emp WHERE sal != 0;

SELECT ename, hiredate, sal FROM emp WHERE sal > 0;

HAVING Clause The HAVING clause filters selected rows only after all rows have been fetched. Using a WHERE clause helps reduce overheads in sorting, summing, etc. HAVING clauses should only be used when columns with summary operations applied to them are restricted by the clause. Given Query SELECT d.dname, AVG (e.sal) FROM emp e, dept d WHERE e.deptno = d.deptno GROUP BY d.dname HAVING dname != 'RESEAECH' AND dname != 'SALES';

Combined Subqueries

Alternative SELECT d.dname, AVG (e.sal) FROM emp e, dept d WHERE e.deptno = d.deptno AND dname != 'RESEAECH' AND dname != 'SALES' GROUP BY d.dname;

Minimize the number of table lookups (subquery blocks) in queries, particularly if your statements include subquery SELECTs or multicolumn UPDATEs. Separate Subqueries SELECT ename FROM emp WHERE sal = (SELECT MAX (sal) FROM lookup) AND comm = (SELECT MAX (comm) FROM lookup);

Combined Subqueries SELECT ename FROM emp WHERE (sal,comm) = (SELECT MAX (sal), MAX(comm) FROM lookup);

EXISTS, NOT IN, Table Joins

Consider the alternatives EXISTS, IN and table joins when doing multiple table joins. Non of these are consistently faster; it depends on your data. SELECT ename FROM emp E WHERE EXISTS (SELECT FROM WHERE AND

'X' dept deptno = E.deptno dname = 'ACCOUNTING');

SELECT ename FROM emp E WHERE deptno IN (SELECT deptno FROM dept WHERE deptno = E.deptno AND dname = 'ACCOUNTING'); SELECT ename FROM dept D, emp E WHERE E.deptno = D.deptno AND D.dname = 'ACCOUNTING';

DISTINCT

Avoid joins that require the DISTINCT qualifier on the SELECT list in queries which are used to determine information at the owner end of a one-to-many relationship. The DISTINCT operator causes Oracle to fetch all rows satisfying the table join and then sort and filter out duplicate values. EXISTS is a faster alternative, because the Oracle optimize realizes when the subquery has been satisfied once, there is no need to proceed further and the next matching row can be fetched. Given Query SELECT DISTINCT d.deptno, d.dname FROM dept D, emp E WHERE D.deptno = E.deptno;

Alternative SELECT d.deptno, d.dname FROM dept D WHERE EXISTS (SELECT 'X' FROM emp E WHERE E.deptno = D.deptno);

UNION ALL

Consider whether a UNION ALL will suffice in place of a UNION. The UNION clause forces all rows returned by each portion of the UNION to be sorted and merged and duplicates t be filtered before the first row is returned. A UNION ALL simply returns all rows including duplicates and does not have to perform any sort, merge or filter. If your tables are mutually exclusive (include no duplicate records), or you don't care if duplicates are returned, the UNION ALL is much more efficient. UNION SELECT acct, balance FROM debit WHERE trandate = '31-DEC-95' UNION SELECT acct, balance FROM credit WHERE trandate = '31-DEC-95';

UNION ALL SELECT acct, balance FROM debit WHERE trandate = '31-DEC-95' UNION ALL SELECT acct, balance FROM credit WHERE trandate = '31-DEC-95';

DECODE

Consider using DECODE to avoid having to scan the same rows repetitively or join the same table repetitively. Note, DECODE is not necessarily faster as it depends on your data and the complexity of the resulting query. Also, using DECODE requires you to change your code when new values are allowed in the field. SELECT COUNT(*) FROM emp WHERE status = 'Y' AND ename LIKE 'SMITH%'; ---------SELECT COUNT(*) FROM emp WHERE status = 'N' AND ename LIKE 'SMITH%'; SELECT COUNT(DECODE(status, 'Y', 'X', NULL)) Y_count, COUNT(DECODE(status, 'N', 'X', NULL)) N_count FROM emp WHERE ename LIKE 'SMITH%';

Anti Joins

An anti-join is used to return rows from a table that that are present in another table. It might b used for example between DEPT and EMP to return only those rows in DEPT that didn't join to anything in EMP;

SELECT * FROM dept WHERE deptno NOT IN (SELECT deptno FROM EMP); SELECT FROM WHERE AND

dept.* dept, emp dept.deptno = emp.deptno (+) emp.ROWID IS NULL;

SELECT * FROM dept WHERE NOT EXISTS (SELECT NULL FROM emp WHERE emp.deptno = dept.deptno);

Full

Outer Joins

Normally, an outer join of table A to table B would return every record in table A, and if it had a mate in table B, that would be returned as well. Every row in table A would be output, but some rows of table B might not appear in the result set. A full outer join would return ebery row in table A, as well as every row in table B. The syntax for a full outer join is new in Oracle 9i, but it is a syntactic convenience, it is possible to produce full outer joins sets using conventional SQL. update emp set deptno = 9 where deptno = 10; commit; Conventional SQL

New Syn

SELECT empno, ename, dept.deptno, dname FROM emp, dept WHERE emp.deptno(+) = dept.deptno UNION ALL SELECT empno, ename, emp.deptno, NULL FROM emp, dept WHERE emp.deptno = dept.deptno(+) AND dept.deptno IS NULL ORDER BY 1,2,3,4;

SELECT empno, ename, NVL(dept.deptno,emp.de FROM emp FULL OUTER JOIN de (emp.deptno = dept.dept ORDER BY 1,2,3,4;

EMPNO ---------7369 7499 7521 7566 7654 7698 7782 7788 7839 7844 7876 7900 7902 7934

EMPNO ---------7369 7499 7521 7566 7654 7698 7782 7788 7839 7844 7876 7900 7902 7934

ENAME DEPTNO DNAME ---------- ---------- -------------SMITH 20 RESEARCH ALLEN 30 SALES WARD 30 SALES JONES 20 RESEARCH MARTIN 30 SALES BLAKE 30 SALES CLARK 9 SCOTT 20 RESEARCH KING 9 TURNER 30 SALES ADAMS 20 RESEARCH JAMES 30 SALES FORD 20 RESEARCH MILLER 9 10 ACCOUNTING 40 OPERATIONS

ENAME DEP ---------- ------SMITH ALLEN WARD JONES MARTIN BLAKE CLARK SCOTT KING TURNER ADAMS JAMES FORD MILLER

Inline VIEWS The inline view is a construct in Oracle SQL where you can place a query in the SQL FROM, clause, just as if the query was a table name.

OK, so why use the complicated select in the first place? Why not just create the view? Well, on good reason is that creating a view gives you another database object to maintain, and adds more complexity to your system. By placing the view "inside" your main select, you have all of the code needed to support the query in one place.

The Power of Inline Views Overview The inline view is a construct in Oracle SQL where you can place a query in the SQL FROM, clause, just as if the query was a table name. OK, so why use the complicated select in the first place? Why not just create the view? Well, one good reason is that creating a view gives you another database object to maintain, and adds more complexity to your system. By placing the view "inside" your main select, you have all of the code needed to support the query in one place. If you have a query as the following ... SELECT FROM WHERE AND

a table id = :id b = (SELECT MAX (b) FROM table WHERE id = :id)

... it can be worth to check if an inline view, instead of the subquery will be faster.

Example 1 (Replace Subquery for MAX) With Subquery CREATE TABLE test (id INT, height INT, acc_date DATE); INSERT INTO test (id, height, acc_date) SELECT MOD(ROWNUM,1000), DBMS_RANDOM.RANDOM, SYSDATE-1000+DBMS_RANDOM.VALUE(0,1000) FROM all_objects; 6357 rows created.

COMMIT; CREATE INDEX test_idx on test (id, acc_date, height); Index created. ANALYZE FOR FOR FOR

TABLE test COMPUTE STATISTICS TABLE ALL INDEXES ALL INDEXED COLUMNS;

Table analyzed. alter session set timed_statistics=true; alter session set sql_trace=true; VARIABLE b1 NUMBER exec :b1 := 10 ALTER SESSION SET TIMED_STATISTICS=TRUE; ALTER SESSION SET SQL_TRACE=TRUE; SELECT from WHERE AND

max(height) test id = :b1 acc_date = (SELECT MAX(acc_date) FROM test WHERE id = :b1);

MAX(HEIGHT) ----------1480603530 Elapsed: 00:00:00.12 Execution Plan ---------------------------------------------------------0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=17) 1 0 SORT (AGGREGATE) 2 1 INDEX (RANGE SCAN) OF 'TEST_IDX' (NON-UNIQUE) (Cost=2 Card=1 Bytes=17) 3 2 SORT (AGGREGATE) 4 3 FIRST ROW (Cost=2 Card=6 Bytes=60) 5 4 INDEX (RANGE SCAN (MIN/MAX)) OF 'TEST_IDX' (NON-UNIQUE) (Cost=2 Card=1060) tkprof gek1_ora_16520.trc gek1_ora_16520.out explain=scott/tiger sort=exeela sys=no call count rows ------- --------------Parse 1

cpu

elapsed

disk

query

current

-------- ---------- ---------- ---------- ---------0.00

0.00

0

0

0

0 Execute 1 0 Fetch 2 1 ------- --------------total 4 1

0.00

0.00

0

2

0

0.00

0.00

0

2

0

-------- ---------- ---------- ---------- ---------0.00

0.00

0

4

0

With Inline View VARIABLE b1 NUMBER exec :b1 := 10 SELECT height FROM (SELECT FROM WHERE ORDER BY WHERE ROWNUM =

height test id = :b1 id DESC, acc_date DESC, height DESC) 1;

HEIGHT ---------1480603530 Execution Plan ---------------------------------------------------------0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=13) 1 0 COUNT (STOPKEY) 2 1 VIEW (Cost=2 Card=6 Bytes=78) 3 2 INDEX (RANGE SCAN DESCENDING) OF 'TEST_IDX' (NON-UNIQUE) (Cost=2 Card=6 Bytes=102) tkprof gek1_ora_16521.trc gek1_ora_16521.out explain=scott/tiger sort=exeela sys=no call count rows ------- --------------Parse 1 0 Execute 1 0 Fetch 2 1 ------- --------------total 4 1

cpu

elapsed

disk

query

current

-------- ---------- ---------- ---------- ---------0.03

0.06

2

41

0

0.00

0.00

0

0

0

0.00

0.00

0

2

0

-------- ---------- ---------- ---------- ---------0.03

0.06

2

43

0

Example 2 (Replace Subquery for MAX) Original Query from a trace session: SELECT switch_time,rat_id FROM tariff WHERE effdate = (SELECT MAX(effdate) FROM tariff WHERE effdate <= TRUNC(:b1) AND weekday = :b2 AND t_id = :b3) AND TO_CHAR(switch_time,'HH24:MI') <= TO_CHAR(:b1,'HH24:MI') AND weekday = :b2 AND t_id = :b3 ORDER BY TO_CHAR(switch_time,'HH24:MI') DESC

With Subquery alter session set timed_statistics=true; select value from v$parameter where name = 'user_dump_dest'; alter session set sql_trace=true; VARIABLE b1 VARCHAR2(19) exec :b1 := '07.04.1999:13:30:31' VARIABLE b2 NUMBER exec :b2 := 2 VARIABLE b3 NUMBER exec :b3 := 317 SELECT switch_time, rat_id FROM tariff WHERE effdate = (SELECT MAX(effdate) FROM tariff WHERE effdate <= TRUNC(TO_DATE(:b1,'DD.MM.YYYY:HH24:MI:SS')) AND weekday = :b2 AND T_ID = :b3) AND TO_CHAR(switch_time,'HH24:MI') <= TO_CHAR(TO_DATE(:b1,'DD.MM.YYYY:HH24:MI:SS'),'HH24:MI') AND weekday = :b2 AND t_id = :b3 ORDER BY TO_CHAR(switch_time,'HH24:MI') DESC; SWITCH_TI RAT_ID --------- ---------01-JAN-98 3 01-JAN-98 1 Execution Plan ---------------------------------------------------------0 SELECT STATEMENT Optimizer=CHOOSE (Cost=4 Card=1 Bytes=21) 1 0 SORT (ORDER BY) (Cost=4 Card=1 Bytes=21)

2 1 FILTER 3 2 TABLE ACCESS (FULL) OF 'TARIFF' (Cost=2 Card=1 Bytes=21) 4 3 SORT (AGGREGATE) 5 4 FILTER 6 5 INDEX (RANGE SCAN) OF 'PK_TARIFF' (UNIQUE) (Cost=2 Card=1 Bytes=12) TKPROF: tkprof xyz.trc xyz.out explain=user/pwd sort=exeela sys=no call count rows ------- --------------Parse 1 0 Execute 1 0 Fetch 2 4 ------- --------------total 4 4

cpu

elapsed

disk

query

current

-------- ---------- ---------- ---------- ---------0.00

0.00

0

0

0

0.00

0.00

0

0

0

0.01

0.00

0

38

8

-------- ---------- ---------- ---------- ---------0.01

0.00

0

38

8

Misses in library cache during parse: 0 Optimizer goal: CHOOSE

With Inline View VARIABLE b1 VARCHAR2(19) exec :b1 := '07.04.2005:13:30:31' VARIABLE b2 NUMBER exec :b2 := 2 VARIABLE b3 NUMBER exec :b3 := 317 SELECT switch_time, rat_iD FROM (SELECT switch_time, rat_id FROM tariff WHERE effdate <= TRUNC(TO_DATE(:b1,'DD.MM.YYYY:HH24:MI:SS')) AND weekday = :b2 AND t_id = :b3 ORDER BY effdate DESC) WHERE TO_CHAR(switch_time,'HH24:MI') <= TO_CHAR(TO_DATE(:b1,'DD.MM.YYYY:HH24:MI:SS'),'HH24:MI'); SWITCH_TI RAT_ID --------- ---------01-JAN-98 3 01-JAN-98 1

Execution Plan ---------------------------------------------------------0 SELECT STATEMENT Optimizer=CHOOSE (Cost=4 Card=1 Bytes=22) 1 0 VIEW (Cost=4 Card=1 Bytes=22) 2 1 SORT (ORDER BY) (Cost=4 Card=1 Bytes=21) 3 2 FILTER 4 3 TABLE ACCESS (BY INDEX ROWID) OF 'TARIFF' (Cost=2 Card=1 Bytes=21) 5 4 INDEX (RANGE SCAN) OF 'PK_TARIFF' (UNIQUE) (Cost=2 Card=1) TKPROF: tkprof xyz.trc xyz.out explain=user/pwd sort=exeela sys=no call count rows ------- --------------Parse 1 0 Execute 1 0 Fetch 2 4 ------- --------------total 4 4

cpu

elapsed

disk

query

current

-------- ---------- ---------- ---------- ---------0.00

0.00

0

0

0

0.00

0.00

0

0

0

0.00

0.00

0

19

4

-------- ---------- ---------- ---------- ---------0.00

0.00

0

19

4

Misses in library cache during parse: 0 Optimizer goal: CHOOSE

Example 3 (cannot have join with CONNECT BY) Have you ever tried to join to a hierarchical query (a query using CONNECT BY and PRIOR) only to get this message: ORA-01437: cannot have join with CONNECT BY One of the limitations of hierarchical queries is that you cannot join to them. However, there are often times you would like to join to them anyway. For instance, if the hierarchy table only has surrogate keys, and you would like to display the real value. This tip shows how you can use "Inline Views" to join tables to a hierarchical query. SELECT level, LPAD(' ',2*level-2)||ename ename, empno, mgr, dept.deptno, dept.dname FROM emp, dept WHERE emp.deptno = dept.deptno CONNECT BY PRIOr empno = mgr START WITH empno = 7839;

ORA-01437: cannot have join with CONNECT BY SELECT E.emplevel, SUBSTR(E.ename,1,15) "ENAME", E.empno, dept.deptno, dept.dname FROM dept, (SELECT level emplevel, LPAD(' ',2*level-2)||ename ename, empno, mgr, deptno FROM emp CONNECT BY PRIOR empno = mgr START WITH empno = 7839) E WHERE E.deptno = dept.deptno / EMPLEVEL ENAME EMPNO DEPTNO DNAME ---------- --------------- ---------- ---------- -------------1 KING 7839 10 ACCOUNTING 2 CLARK 7782 10 ACCOUNTING 3 MILLER 7934 10 ACCOUNTING 2 JONES 7566 20 RESEARCH 3 SCOTT 7788 20 RESEARCH 4 ADAMS 7876 20 RESEARCH 3 FORD 7902 20 RESEARCH 4 SMITH 7369 20 RESEARCH 2 BLAKE 7698 30 SALES 3 ALLEN 7499 30 SALES 3 WARD 7521 30 SALES 3 MARTIN 7654 30 SALES 3 TURNER 7844 30 SALES 3 JAMES 7900 30 SALES

Example 3 (ROWNUM 1 Problem) A rownum restriction starting with 1 works: ROWNUM does not work for ranges that don't start at 1. A ROWNUM restriction starting with 1 works: SELECT ROWNUM,ename from emp WHERE ROWNUM BETWEEN 1 and 3 / ROWNUM ---------1 2 3

ENAME ---------SMITH ALLEN WARD

However, if you try to use a range it will not work. For example: SELECT ROWNUM,ename from emp WHERE ROWNUM BETWEEN 2 and 3 / no rows selected

Using an Inline View to get around this limitation: SELECT t1.rn, t1.ename FROM (SELECT ROWNUM rn, ename FROM emp) t1 WHERE t1.rn BETWEEN 2 and 3 / The main trick to this query is the "internal" select statement. This select statement in the from clause, basically does a full query of the table, then returns the values (along with the psuedo-column ROWNUM) to the "outside" query. The outside query can then operate on the results of the internal query. In order to access the internal query's columns from the external query, you need to give the internal query an alias ("t1" highlighted below): This allows you to refer to the columns using the "t1" (highlighted below): Since "ROWNUM" is a psuedo-column and therefore a reserved word, you need to alias that column in the internal query in order to refer to it in the outside query:

Example 4 (ROWNUM and ORDER BY Problem, TOP-N Queries) The following query form is almost wrong: select * from emp where ROWNUM <= 5 order by sal desc; EMPNO DEPTNO ------------------7566 2975 7499 300 7521 500 7654 1400 7369 800

ENAME

JOB

MGR HIREDATE

/* WRONG! */ SAL

COMM

---------- --------- ---------- --------- ---------- ---------JONES ALLEN 30 WARD 30 MARTIN 30 SMITH

MANAGER 20 SALESMAN

7839 02-APR-81 7698 20-FEB-81

1600

SALESMAN

7698 22-FEB-81

1250

SALESMAN

7698 28-SEP-81

1250

CLERK 20

7902 17-DEC-80

The users intention was most likely to get the the top-five paid people - a top-N query. What the will get is five random records (the first five we happen to hit), sorted by salary. If you use an inline view with the ORDER BY inside the inline view, you get the correct result. select * from (select * from emp order by sal desc) where rownum <= 5; EMPNO DEPTNO ------------------7839 5000 7788

ENAME

JOB

MGR HIREDATE

SAL

COMM

---------- --------- ---------- --------- ---------- ---------KING SCOTT

PRESIDENT 10 ANALYST

17-NOV-81 7566 09-DEC-82

3000 3000 2975

7902 FORD 7566 JONES 7698 BLAKE

2850

20 ANALYST 20 MANAGER 20 MANAGER 30

7566 03-DEC-81 7839 02-APR-81 7839 01-MAY-81

Example 5 (Pagination with ROWNUM) Pagination with ROWNUM can be used to get rows N thru M of a result set. The general form of this is as follows: SELECT * FROM (SELECT a.*, ROWNUM rn FROM (enter your query here) a WHERE ROWNUM <= :MAX_ROW) WHERE rn >= :MIN_ROW; SELECT * FROM (SELECT a.*, ROWNUM rn FROM (SELECT * FROM emp) a WHERE ROWNUM <= 6) WHERE rn >= 2; EMPNO DEPTNO ------------------7499 300 7521 500 7566 2975 7654 1400 7698 2850

ENAME RN ------------------ALLEN 30 WARD 30 JONES MARTIN 30 BLAKE

JOB

MGR HIREDATE

SAL

COMM

--------- ---------- --------- ---------- ---------SALESMAN 2 SALESMAN 3 MANAGER 20 SALESMAN 5 MANAGER 30

7698 20-FEB-81

1600

7698 22-FEB-81

1250

7839 02-APR-81 4

7698 28-SEP-81 7839 01-MAY-81

6

1250