Correlated subquery execution problem

Question

Hi
In correlated subqueries which query executed first whether the inner query or outer query i.e while oracle processing the query
give me example
thanx
asp

Answer 1

I would say the outer query would be executed first as the inner query is dependant upon that but I guess in some circumstances the CBO will turn this rule on its head
SQL> select ename from emp
  2  where exists ( select null from dept where dept.deptno = emp.deptno )
  3  /
ENAME
SMITH
ALLEN
WARD
JONES
MARTIN
BLAKE
CLARK
SCOTT
KING
TURNER
ADAMS
ENAME
JAMES
FORD
MILLER
14 rows selected.
Execution Plan
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=4 Card=14 Bytes=16
          8)
   1    0   NESTED LOOPS (SEMI) (Cost=4 Card=14 Bytes=168)
   2    1     TABLE ACCESS (FULL) OF 'EMP' (TABLE) (Cost=3 Card=14 Byt
          es=126)
   3    1     INDEX (UNIQUE SCAN) OF 'PK_DEPT' (INDEX (UNIQUE)) (Cost=
          1 Card=7 Bytes=21)
  1  select ename from emp
  2* where not exists ( select null from dept where dept.deptno = emp.deptno )
SQL> /
no rows selected
Execution Plan
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=4 Card=1 Bytes=12)
   1    0   NESTED LOOPS (ANTI) (Cost=4 Card=1 Bytes=12)
   2    1     TABLE ACCESS (FULL) OF 'EMP' (TABLE) (Cost=3 Card=14 Byt
          es=126)
   3    1     INDEX (UNIQUE SCAN) OF 'PK_DEPT' (INDEX (UNIQUE)) (Cost=
          1 Card=7 Bytes=21)