oracle中exists连接两个表,IN、EXISTS、多表连接，哪个速度更快

本帖最后由 ghsau 于 2011-10-11 23:39 编辑

请看下面三个执行计划(Oracle10g)

1. 用IN

SQL> select ename from emp e where e.deptno in (select d.deptno from dept d where d.dname='SALES');

PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------

Plan hash value: 351108634

----------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 5 | 105 | 5 (0)| 00:00:01 |

| 1 | NESTED LOOPS | | 5 | 105 | 5 (0)| 00:00:01 |

| 2 | TABLE ACCESS FULL | EMP | 14 | 126 | 3 (0)| 00:00:01 |

|* 3 | TABLE ACCESS BY INDEX ROWID | DEPT | 1 | 12 | 1 (0)| 00:00:01 |

|* 4 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| 00:00:01 |

----------------------------------------------------------------------------------------

PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

3 - filter("D"."DNAME"='SALES')

4 - access("E"."DEPTNO"="D"."DEPTNO")

2. 用EXISTS

SQL> select e.ename from emp e where exists (select 1 from dept d where e.deptno=d.deptno and d.dname='SALES');

PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------

Plan hash value: 90266402

----------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 5 | 105 | 5 (0)| 00:00:01 |

| 1 | NESTED LOOPS SEMI | | 5 | 105 | 5 (0)| 00:00:01 |

| 2 | TABLE ACCESS FULL | EMP | 14 | 126 | 3 (0)| 00:00:01 |

|* 3 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 12 | 1 (0)| 00:00:01 |

|* 4 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| 00:00:01 |

----------------------------------------------------------------------------------------

PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

3 - filter("D"."DNAME"='SALES')

4 - access("E"."DEPTNO"="D"."DEPTNO")

3. 用连接

SQL> select e.ename from emp e join dept d on e.deptno=d.deptno where d.dname='SALES';

PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------

Plan hash value: 351108634

----------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 5 | 105 | 5 (0)| 00:00:01 |

| 1 | NESTED LOOPS | | 5 | 105 | 5 (0)| 00:00:01 |

| 2 | TABLE ACCESS FULL | EMP | 14 | 126 | 3 (0)| 00:00:01 |

|* 3 | TABLE ACCESS BY INDEX ROWID | DEPT | 1 | 12 | 1 (0)| 00:00:01 |

|* 4 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| 00:00:01 |

----------------------------------------------------------------------------------------

PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

3 - filter("D"."DNAME"='SALES')

4 - access("E"."DEPTNO"="D"."DEPTNO")

按我来看，正常的执行效率应该是3>2>1，但是竟然产生了三个一模一样的执行计划，我原来看过的一个文档是这样说得：

其实我们区分in和exists主要是造成了驱动顺序的改变(这是性能变化的关键)，如果是exists，那么以外层表为驱动表，先被访问，如果是IN，那么先执行子查询，所以我们会以驱动表的快速返回为目标，那么就会考虑到索引及结果集的关系了复制代码这是其一，在执行计划里没有体现其二：我们通常是自己选择优化器，还是用Oracle默认的优化器呢？

其三：Oracle10g有三种优化器，CHOOSE/COST/RULE，默认为CHOOSE，当表被analyze过时，选择COST，否则选择RULE，但是ORACLE10g默认分析表的，这里比较迷惑，求解

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