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，假如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
?????
----------------------------------------------------------
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)