转自http://www.2cto.com/database/201310/251176.html
对于in和exists、not in和not exists还是有很多的人有疑惑，更有甚者禁用not in，所有的地方都要用not exists，它真的高效吗？
【实验1 in和exists原理及性能比较】
准备数据
create table test1 as select * from dba_objects where rownum <=1000;
create table test2 as select * from dba_objects;
exec dbms_stats.gather_table_stats(user,'test1');
exec dbms_stats.gather_table_stats(user,'test2');
set autotrace traceonly
in 查询
select * from test1 t1 where t1.object_id in (select t2.object_id from test2 t2);
执行计划
----------------------------------------------------------
Plan hash value: 3819917785
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1000 | 90000 | 307 (1) | 00:00:04 |
|* 1 | HASH JOIN SEMI | | 1000 | 90000 | 307 (1) | 00:00:04 |
| 2 | TABLE ACCESS FULL | TEST1 | 1000 | 85000 | 6 (0) | 00:00:01 |
| 3 | TABLE ACCESS FULL | TEST2 | 73119 | 357K| 301 (1) | 00:00:04 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
98 consistent gets
0 physical reads
0 redo size
50936 bytes sent via SQL*Net to client
1226 bytes received via SQL*Net from client
68 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1000 rows processed
exists 查询
select * from test1 t1 where exists(select 1 from test2 t2 where t1.object_id=t2.object_id);
执行计划
----------------------------------------------------------
Plan hash value: 3819917785
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1000 | 90000 | 307 (1) | 00:00:04 |
|* 1 | HASH JOIN SEMI | | 1000 | 90000 | 307 (1) | 00:00:04 |
| 2 | TABLE ACCESS FULL | TEST1 | 1000 | 85000 | 6 (0) | 00:00:01 |
| 3 | TABLE ACCESS FULL | TEST2 | 73119 | 357K| 301 (1) | 00:00:04 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
98 consistent gets
0 physical reads
0 redo size
50936 bytes sent via SQL*Net to client
1226 bytes received via SQL*Net from client
68 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1000 rows processed
结论：
在oracle 11g中，in和exists 其实是一样的，原理就是两张表做HASH JOIN SEMI。也可以通过10053事件看到两条sql语句最终转换成同一条sql。
【实验2 not in和not exists原理及性能比较】
not exists 比 not in 效率高的例子（按照转载文章实验，执行计划和文章不符，结果是效率相同，可能是由于本人使用版本11g高于原文章缘故）
保持test1 和 test2 数据不变，分别是 1000、70000+
select count(*) from test1 where object_id not in (select object_id from test2);
select count(*) from test1 t1 where not exists(select 1 from test2 t2 where t1.object_id=t2.object_id);
执行计划相同，此处就省略了。
执行计划相同；效率一样
not in 比 not exists 效率高的例子（依然和转载文章结果不符，结果还是效率相同，后来我用hint改变了not in的执行计划才能显示出not in的优势）
准备数据
创建表t1和t2，结构和test1、test2一样，但是t1数据量为5条，t2数据量为20W+
select count(*) from t1 where object_id not in (select /*+ no_unnest */ object_id from t2);
--注意：如果不用hint来改变执行计划，两个语句仍然是一样的执行计划；
执行计划
----------------------------------------------------------
Plan hash value: 59119136
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 755 (1)| 00:00:10 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | FILTER | | | | | |
| 3 | TABLE ACCESS FULL| T1 | 5 | 15 | 3 (0)| 00:00:01 |
|* 4 | TABLE ACCESS FULL| T2 | 2 | 10 | 301 (1)| 00:00:04 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter( NOT EXISTS (SELECT /*+ NO_UNNEST */ 0 FROM "T2" "T2"
WHERE LNNVL("OBJECT_ID"<>:B1)))
4 - filter(LNNVL("OBJECT_ID"<>:B1))
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
23 consistent gets
0 physical reads
0 redo size
522 bytes sent via SQL*Net to client
500 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
select count(*) from t1 where not exists (select 1 from t2 where t1.object_id=t2.object_id);
执行计划
----------------------------------------------------------
Plan hash value: 1513027705
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 8 | 2376 (1)| 00:00:29 |
| 1 | SORT AGGREGATE | | 1 | 8 | | |
|* 2 | HASH JOIN ANTI | | 1 | 8 | 2376 (1)| 00:00:29 |
| 3 | TABLE ACCESS FULL| T1 | 5 | 15 | 3 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL| T2 | 584K| 2856K| 2371 (1)| 00:00:29 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
8599 consistent gets
0 physical reads
0 redo size
522 bytes sent via SQL*Net to client
500 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
结论
在11g版本中，数据量如我制造类似情况下，in和exists，not in和not exists的执行计划已经基本一致了，更倾向于使用HASH JOIN，但是当外表非常小，内表非常大的情况下，通过hint改变执行计划，filter的性能可以更优于HASH JOIN，也说明了not in不一定性能比not exists 差。