Question

I have a set of conditions in my where clause like

WHERE  d.attribute3 = 'abcd*'   AND x.STATUS != 'P'  AND x.STATUS != 'J'  AND x.STATUS != 'X'  AND x.STATUS != 'S'  AND x.STATUS != 'D'  AND CURRENT_TIMESTAMP - 1 < x.CREATION_TIMESTAMP 

Which of these conditions will be executed first? I am using oracle.

Will I get these details in my execution plan? (I do not have the authority to do that in the db here, else I would have tried)

Answer 1

Are you sure you ‘don’t have the authority’ to see an execution plan? What about using AUTOTRACE?

SQL> set autotrace on SQL> select * from emp   2  join dept on dept.deptno = emp.deptno   3  where emp.ename like 'K%'   4  and dept.loc like 'l%'   5  /  no rows selected   Execution Plan ----------------------------------------------------------  ---------------------------------------------------------------------------------- | Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| ---------------------------------------------------------------------------------- |   0 | SELECT STATEMENT             |              |     1 |    62 |     4   (0)| |   1 |  NESTED LOOPS                |              |     1 |    62 |     4   (0)| |*  2 |   TABLE ACCESS FULL          | EMP          |     1 |    42 |     3   (0)| |*  3 |   TABLE ACCESS BY INDEX ROWID| DEPT         |     1 |    20 |     1   (0)| |*  4 |    INDEX UNIQUE SCAN         | SYS_C0042912 |     1 |       |     0   (0)| ----------------------------------------------------------------------------------  Predicate Information (identified by operation id): ---------------------------------------------------     2 - filter('EMP'.'ENAME' LIKE 'K%' AND 'EMP'.'DEPTNO' IS NOT NULL)    3 - filter('DEPT'.'LOC' LIKE 'l%')    4 - access('DEPT'.'DEPTNO'='EMP'.'DEPTNO') 

As you can see, that gives quite a lot of detail about how the query will be executed. It tells me that:

• the condition ’emp.ename like ‘K%” will be applied first, on the full scan of EMP
• then the matching DEPT records will be selected via the index on dept.deptno (via the NESTED LOOPS method)
• finally the filter ‘dept.loc like ‘l%’ will be applied.

This order of application has nothing to do with the way the predicates are ordered in the WHERE clause, as we can show with this re-ordered query:

SQL> select * from emp   2  join dept on dept.deptno = emp.deptno   3  where dept.loc like 'l%'   4  and emp.ename like 'K%';  no rows selected   Execution Plan ----------------------------------------------------------  ---------------------------------------------------------------------------------- | Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| ---------------------------------------------------------------------------------- |   0 | SELECT STATEMENT             |              |     1 |    62 |     4   (0)| |   1 |  NESTED LOOPS                |              |     1 |    62 |     4   (0)| |*  2 |   TABLE ACCESS FULL          | EMP          |     1 |    42 |     3   (0)| |*  3 |   TABLE ACCESS BY INDEX ROWID| DEPT         |     1 |    20 |     1   (0)| |*  4 |    INDEX UNIQUE SCAN         | SYS_C0042912 |     1 |       |     0   (0)| ----------------------------------------------------------------------------------  Predicate Information (identified by operation id): ---------------------------------------------------     2 - filter('EMP'.'ENAME' LIKE 'K%' AND 'EMP'.'DEPTNO' IS NOT NULL)    3 - filter('DEPT'.'LOC' LIKE 'l%')    4 - access('DEPT'.'DEPTNO'='EMP'.'DEPTNO')