oracle sample是怎么扫描

发布网友发布时间：2022-04-10 20:31

共1个回答

热心网友时间：2022-04-10 22:01

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)