ORACLE回表（三）

来源：互联网发布：李世民杀兄知乎编辑：程序博客网时间：2024/06/07 07:18

http://www.itpub.net/thread-1340290-1-1.html

“回表”一般就是指执行计划里显示的“TABLE ACCESS BY INDEX ROWID”。
例如select的字段里有索引不包含的列，
SQL> select ename from emp  where empno =1;

Execution Plan
----------------------------------------------------------
Plan hash value: 4066871323

----------------------------------------------------------------------
| Id  | Operation                                       | Name | Rows  | Bytes | Cost  |
----------------------------------------------------------------------
| 0 | SELECT STATEMENT                         |              |    1 |    8 |    1 |
| 1 |  TABLE ACCESS BY INDEX ROWID    | EMP     |    1 |    8 |    1 |
|*  2 | INDEX UNIQUE SCAN                    | PK_EMP |    1 |    |    1 |
----------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - access("EMPNO"=1)

再例如，虽然只查询索引里的列，但是需要回表过滤掉其他行。
SQL> select empno from emp  where empno =1 and ename='hao';

Execution Plan
----------------------------------------------------------
Plan hash value: 4066871323

----------------------------------------------------------------------
| Id  | Operation                                   | Name    | Rows  | Bytes | Cost  |
----------------------------------------------------------------------
| 0 | SELECT STATEMENT                     |             |    1 |    8 |    1 |
|*  1 |  TABLE ACCESS BY INDEX ROWID| EMP      |    1 |    8 |    1 |
|*  2 | INDEX UNIQUE SCAN                 | PK_EMP |    1 |    |    1 |
----------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("ENAME"='hao')
2 - access("EMPNO"=1)