浅析Oracle 10gR2中优化器对表关联的优化

 在以前的一篇文章中，介绍过额外的关联对SQL的影响是很大的，所以在sql审核的工作中，对于表关联的关注是相当多的，如果冗余能够优化掉表关联，偶都尽量在表设计上做些冗余处理；在10gR2中，发现优化器可以优化掉一些不必要的关联……

9206:
SQL> set autot on
SQL> select count(*)
2 from dept d, emp e
3 where d.deptno = e.deptno;
COUNT(*)
----------
12
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 SORT (AGGREGATE)
2 1 NESTED LOOPS
3 2 TABLE ACCESS (FULL) OF 'EMP'
4 2 INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)

10202
SQL> set autot on
SQL> select count(*)
2 from dept d, emp e
3 where d.deptno = e.deptno;
COUNT(*)
----------
12
ִѐ¼ƻ®
----------------------------------------------------------
Plan hash value: 2083865914
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
|* 2 | TABLE ACCESS FULL| EMP | 12 | 156 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("E"."DEPTNO" IS NOT NULL)

原文链接： http://blog.csdn.net/newhappy2008/article/details/3940790