Order of Execution in WHERE clause

Question

DB Version 10gR2
In a query like
SELECT sal from emp where empid=7891 and empname='JOHN';
What does Oracle evaluate first in the WHERE clause, is it empid=7891 or empname='JOHN'? I couldn't find this in 10G R2 documentation, hence this thread.

Answer 1

Hi,
You need to read the execution plan of a query to see which filtering condition is applied first or last.
For the sample "EMP" table from SCOTT schema, following is clear:
SQL> set autotrace on
SQL> SELECT sal from emp where empno=7891 and ename='JOHN';
no rows selected
Execution Plan
Plan hash value: 2949544139
| Id  | Operation                   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |        |     1 |    14 |     1   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS BY INDEX ROWID| EMP    |     1 |    14 |     1   (0)| 00:00:01 |
|*  2 |   INDEX UNIQUE SCAN         | PK_EMP |     1 |       |     0   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter("ENAME"='JOHN')
   2 - access("EMPNO"=7891)
Statistics
          0  recursive calls
          0  db block gets
          1  consistent gets
          0  physical reads
          0  redo size
        272  bytes sent via SQL*Net to client
        384  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed
SQL>Message was edited by:
Citrus
added complete autotrace output