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1.ORDER BY 索引列
走索引
示例:
12:15:49 SCOTT@orcl> select * from emp order by empno;
已用时间: 00: 00: 00.01
执行计划
----------------------------------------------------------
Plan hash value: 4004826672
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 560 | 2 (0)| 00:00:01 || 1 | TABLE ACCESS BY INDEX ROWID| EMP | 14 | 560 | 2 (0)| 00:00:01 || 2 | INDEX FULL SCAN | EMP_PK | 14 | | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------
显然走了主键索引EMP_PK.
2.order by 索引列a,索引列b。
不走索引
示例:
--查看索引
12:18:48 SCOTT@orcl> select INDEX_NAME,CO