Oracle支持SELECT语句只扫描表的一部分记录。
这篇简单介绍SAMPLE扫描和HINT的关系。
Oracle的文档上描述，当不包含SAMPLE语句的时候，可以使用HINT来指定执行计划，实际上即使包含SAMPLE语句，HINT也是生效的。
SQL> SELECT OBJECT_ID FROM T SAMPLE (1) WHERE WNER = USER;
OBJECT_ID
----------
60607
70958
执行计划
----------------------------------------------------------
Plan hash value: 3630032853
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 22 | 506 | 6 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 22 | 506 | 6 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_T_OWNER | 22 | | 5 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OWNER"=USER@!)
filter(ORA_HASH(ROWID,0,2211694651,'SYS_SAMPLE',0)<42949673)
统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
6 consistent gets
4 physical reads
0 redo size
452 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
SQL> SELECT /*+ FULL(T) */ OBJECT_ID FROM T SAMPLE (1) WHERE WNER = USER;
未选定行
执行计划
----------------------------------------------------------
Plan hash value: 2767392432
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 22 | 506 | 158 (2)| 00:00:02 |
|* 1 | TABLE ACCESS SAMPLE| T | 22 | 506 | 158 (2)| 00:00:02 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OWNER"=USER@!)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
517 consistent gets
430 physical reads
0 redo size
275 bytes sent via SQL*Net to client
374 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
Oracle文档还提到基于BLOCK的SAMPLE扫描，只对全表扫描和快速索引全扫有效，而其他执行计划无效，不过索引扫描提供了一种类似实现方式：SYS_SAMPLE_BLOCK
SQL> SELECT OBJECT_ID FROM T SAMPLE BLOCK (1);
未选定行
执行计划
----------------------------------------------------------
Plan hash value: 2767392432
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 496 | 8432 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS SAMPLE| T | 496 | 8432 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------
统计信息
----------------------------------------------------------
219 recursive calls
0 db block gets
40 consistent gets
0 physical reads
0 redo size
275 bytes sent via SQL*Net to client
374 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
5 sorts (memory)
0 sorts (disk)
0 rows processed
SQL> SELECT OBJECT_ID FROM T SAMPLE BLOCK (1) WHERE OBJECT_ID IS NOT NULL;
未选定行
执行计划
----------------------------------------------------------
Plan hash value: 1022620004
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 496 | 8432 | 2 (0)| 00:00:01 |
|* 1 | INDEX SAMPLE FAST FULL SCAN| IND_T_ID | 496 | 8432 | 2 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID" IS NOT NULL)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
6 consistent gets
0 physical reads
0 redo size
275 bytes sent via SQL*Net to client
374 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
SQL> SELECT /*+ INDEX(T) */ OBJECT_ID FROM T SAMPLE BLOCK (0.01) WHERE OBJECT_ID IS NOT NULL;
OBJECT_ID
----------
627
35358
44369
执行计划
----------------------------------------------------------
Plan hash value: 1376157901
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5 | 85 | 113 (2)| 00:00:02 |
|* 1 | INDEX FULL SCAN | IND_T_ID | 5 | 85 | 113 (2)| 00:00:02 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID" IS NOT NULL AND
ORA_HASH(ROWID,0,533460469,'SYS_SAMPLE_BLOCK',0)<429497)
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
112 consistent gets
0 physical reads
0 redo size
461 bytes sent via SQL*Net to client
385 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
3 rows processed
事实上SAMPLE和HINT二者并不矛盾，绝大部分情况下，Oracle采用的执行计划会同时满足二者的需求。
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