关于css:Oracle-sql语句-minus函数执行效率与join对比

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咱们常常会对 select 后果进行比照，罕用的函数如 minus，那么当两张表数据量较大时，有什么形式能够提高效率呢？

minus 形式
minus 执行打算如下所示，通过排序后再进行比照
select object_id from t2 minus select object_id from t1;
..
2255 rows selected.
Elapsed: 00:00:00.93
Execution Plan

Plan hash value: 1578327006

| 0 | SELECT STATEMENT | | 102K| 2357K| | 1318 (1)| 00:00:16 |
| 1 | MINUS | | | | | | |
| 2 | SORT UNIQUE | | 102K| 1304K| 2024K| 861 (1)| 00:00:11 |
| 3 | TABLE ACCESS FULL | T2 | 102K| 1304K| | 371 (1)| 00:00:05 |
| 4 | SORT UNIQUE | | 82926 | 1052K| 1640K| 457 (1)| 00:00:06 |
| 5 | INDEX FAST FULL SCAN| T1_IDX | 82926 | 1052K| | 60 (0)| 00:00:01 |

Note

dynamic sampling used for this statement (level=2)

Statistics

  9  recursive calls   -- 递归
  0  db block gets
   1681  consistent gets
   1798  physical reads
  0  redo size
  41734  bytes sent via SQL*Net to client
   2173  bytes received via SQL*Net from client
152  SQL*Net roundtrips to/from client
  2  sorts (memory)
  0  sorts (disk)
   2255  rows processed

应用 join 形式代替
尤其数据量越大的状况下，join 形式，不排序，走的 hash 算法，执行游戏效率会更好
alter system flush buffer cache;
select t2.object_id t2_id from t1 right join t2 on t1.object_id=t2.object_id where t1.object_id is null;
2255 rows selected.
Elapsed: 00:00:00.56
Execution Plan

Plan hash value: 4276371593

| 0 | SELECT STATEMENT | | 102K| 2609K| | 653 (1)| 00:00:08 |
|* 1 | HASH JOIN RIGHT ANTI | | 102K| 2609K| 2032K| 653 (1)| 00:00:08 |
| 2 | INDEX FAST FULL SCAN| T1_IDX | 82926 | 1052K| | 60 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL | T2 | 102K| 1304K| | 371 (1)| 00:00:05 |

Predicate Information (identified by operation id):

1 – access(“T1″.”OBJECT_ID”=”T2″.”OBJECT_ID”)
Note

dynamic sampling used for this statement (level=2)

Statistics

  0  recursive calls    -- 没有递归游戏调用
  0  db block gets
   1692  consistent gets
   1534  physical reads
  0  redo size
  41730  bytes sent via SQL*Net to client
   2173  bytes received via SQL*Net from client
152  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
   2255  rows processed

正文完