众所周知，随着不断地进行表记录的DML操作，会不断提高表的高水位线（HWM），DELETE操作之后虽然表的数据删除了，但是并没有降低表的高水位，除非你使用TRUNCATE操作，进行表查询的时候，Oracle会扫表高水位以下的数据块，也就是说，扫描的时间并不会有所减少。所以DELETE删除数据以后并不会提高表的查询效率。
下面通过这个例子，用来解决高水位引起的查询变慢问题：
--例子中测试表占用表空间大小为：128M
SQL> SELECT a.bytes/1024/1024 || 'M' FROM user_segments a WHERE a.segment_name = 'TC_RES_PHY_EQP_PRO_MID_517';
A.BYTES/1024/1024||'M'
-----------------------------------------
128M
--查询一条记录成本为：4357，一致性读为：15730 耗时 0.53 秒
SQL> set autotrace on
SQL> SELECT 1 FROM TC_RES_PHY_EQP_PRO_MID_517 a WHERE a.obj_id = 17202000000001;
1
----------
1
执行计划
----------------------------------------------------------
Plan hash value: 854298875
------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 175 | 2275 | 4357 (2)| 00:00:53 |
|* 1 | TABLE ACCESS FULL| TC_RES_PHY_EQP_PRO_MID_517 | 175 | 2275 | 4357 (2)| 00:00:53 |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("A"."OBJ_ID"=17202000000001)
Note
-----
- dynamic sampling used for this statement (level=2)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
15730 consistent gets
0 physical reads
0 redo size
520 bytes sent via SQL*Net to client
520 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--现在删除大部分数据，只剩下一条测试数据：
SQL> DELETE FROM TC_RES_PHY_EQP_PRO_MID_517 a WHERE a.obj_id <> 17202000000001;
已删除1172857行。
--查询该段占用的表空间仍然为128M
SQL> set autotrace off
SQL> SELECT a.bytes/1024/1024 || 'M' FROM user_segments a WHERE a.segment_name = 'TC_RES_PHY_EQP_PRO_MID_517';
A.BYTES/1024/1024||'M'
-----------------------------------------
128M
SQL> COMMIT;
提交完成。
SQL> SELECT a.bytes/1024/1024 || 'M' FROM user_segments a WHERE a.segment_name = 'TC_RES_PHY_EQP_PRO_MID_517';
A.BYTES/1024/1024||'M'
-----------------------------------------
128M
--查询一条记录消耗的成本为：4316，一致性读为：15730 耗时 0.52 秒
SQL> set autotrace on
SQL> SELECT 1 FROM TC_RES_PHY_EQP_PRO_MID_517 a WHERE a.obj_id = 17202000000001;
1
----------
1
执行计划
----------------------------------------------------------
Plan hash value: 854298875
------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 4316 (1)| 00:00:52 |
|* 1 | TABLE ACCESS FULL| TC_RES_PHY_EQP_PRO_MID_517 | 1 | 13 | 4316 (1)| 00:00:52 |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("A"."OBJ_ID"=17202000000001)
Note
-----
- dynamic sampling used for this statement (level=2)
统计信息
---------------------------------------