Oracle not exists的等价写法

最新推荐文章于 2025-07-02 10:17:21 发布

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最新推荐文章于 2025-07-02 10:17:21 发布

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分类专栏： Oracle SQL优化

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本文通过实验对比了 SQL 中 NOT EXISTS 子句与 LEFT JOIN 加上 ISNULL 的用法, 发现两者在查询结果及执行计划上完全等价, 均用于实现外连接中返回左表中不存在于右表的数据。

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not exists可以改为left join + is null，可以看到改写前后执行计划一样，消耗资源一样，说明完全等价。

SQL> drop table test purge;

SQL> drop table test1 purge;
SQL> create table test as select * from dba_objects;
SQL> create table test1 as select * from dba_objects;
SQL> delete from test1 where rownum < 10;
SQL> commit;

SQL> select count(1) from test t where not exists(
select 1 from test1 t1 where t1.object_id=t.object_id
);
COUNT(1)
----------
11
SQL> select count(1) from test t,test1 t1 where t.object_id=t1.object_id(+)
and t1.object_id is null;
COUNT(1)
----------
11
SQL> select * from test t where not exists(
select 1 from test1 t1 where t1.object_id=t.object_id
)
minus
select t.* from test t,test1 t1 where t.object_id=t1.object_id(+)
and t1.object_id is null;
未选定行

SQL> select t.* from test t,test1 t1 where t.object_id=t1.object_id(+)
and t1.object_id is null
minus
select * from test t where not exists(
select 1 from test1 t1 where t1.object_id=t.object_id
);
未选定行

SQL> set autotrace traceonly
SQL> select t.* from test t where not exists(
select 1 from test1 t1 where t1.object_id=t.object_id
);
已选择11行。
执行计划
----------------------------------------------------------
Plan hash value: 2726816538
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 72877 | 15M| | 1109 (1)| 00:00:16 |
|* 1 | HASH JOIN RIGHT ANTI| | 72877 | 15M| 1520K| 1109 (1)| 00:00:16 |
| 2 | TABLE ACCESS FULL | TEST1 | 61874 | 785K| | 196 (1)| 00:00:03 |
| 3 | TABLE ACCESS FULL | TEST | 72877 | 14M| | 197 (2)| 00:00:03 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."OBJECT_ID"="T"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
统计信息
----------------------------------------------------------
7 recursive calls
0 db block gets
1142 consistent gets
0 physical reads
0 redo size
1577 bytes sent via SQL*Net to client
337 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
11 rows processed
SQL> select t.* from test t,test1 t1 where t.object_id=t1.object_id(+)
2 and t1.object_id is null;
已选择11行。
执行计划
----------------------------------------------------------
Plan hash value: 2726816538
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 72877 | 15M| | 1109 (1)| 00:00:16 |
|* 1 | HASH JOIN RIGHT ANTI| | 72877 | 15M| 1520K| 1109 (1)| 00:00:16 |
| 2 | TABLE ACCESS FULL | TEST1 | 61874 | 785K| | 196 (1)| 00:00:03 |
| 3 | TABLE ACCESS FULL | TEST | 72877 | 14M| | 197 (2)| 00:00:03 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T"."OBJECT_ID"="T1"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
统计信息
----------------------------------------------------------
7 recursive calls
0 db block gets
1142 consistent gets
0 physical reads
0 redo size
1577 bytes sent via SQL*Net to client
337 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
11 rows processed