1.
EXISTS的执行流程
select * from t1 where exists ( select null from t2 where y = x )
可以理解为:
for x in ( select * from t1 )
loop
if ( exists ( select null from t2 where y = x.x )
then
OUTPUT THE RECORD
end if
end loop
对于in 和 exists的性能区别:
如果子查询得出的结果集记录较少，主查询中的表较大且又有索引时应该用in,反之如果外层的主查询记录较少，子查询中的表大，又有索引时使用exists。
其实我们区分in和exists主要是造成了驱动顺序的改变（这是性能变化的关键），如果是exists，那么以外层表为驱动表，先被访问，如果是IN，那么先执行子查询，所以我们会以驱动表的快速返回为目标，那么就会考虑到索引及结果集的关系了
另外IN时不对NULL进行处理
如：
select 1 from dual where null in (0,1,2,null)
为空
2.NOT IN 与NOT EXISTS:
NOT EXISTS的执行流程
select .....
from rollup R
where not exists ( select 'Found' from title T
where R.source_id = T.Title_ID);
可以理解为:
for x in ( select * from rollup )
loop
if ( not exists ( that query ) ) then
OUTPUT
end if;
end;
注意:NOT EXISTS 与 NOT IN 不能完全互相替换，看具体的需求。如果选择的列可以为空，则不能被替换。
例如下面语句，看他们的区别：
select x,y from t;
x y
------ ------
1 3
3 1
1 2
1 1
3 1
5
select * from t where x not in (select y from t t2 )
no rows
select * from t where not exists (select null from t t2
where t2.y=t.x )
x y
------ ------
5 NULL
所以要具体需求来决定
对于not in 和 not exists的性能区别：
not in 只有当子查询中，select 关键字后的字段有not null约束或者有这种暗示时用not in,另外如果主查询中表大，子查询中的表小但是记录多，则应当使用not in,并使用anti hash join.
如果主查询表中记录少，子查询表中记录多，并有索引，可以使用not exists,另外not in最好也可以用/*+ HASH_AJ */或者外连接+is null
NOT IN 在基于成本的应用中较好
比如:
select .....
from rollup R
where not exists ( select 'Found' from title T
where R.source_id = T.Title_ID);
改成（佳）
select ......
from title T, rollup R
where R.source_id = T.Title_id(+)
and T.Title_id is null;
或者（佳）
sql> select /*+ HASH_AJ */ ...
from rollup R
where ource_id NOT IN ( select ource_id
from title T
where ource_id IS NOT NULL )
建两个表，outer大表，外层表/inner小表，内层表
SQL> create table inner as select rownum as id from dba_tables;
Table created.
SQL> create table outer as select rownum*2 as id from dba_objects;
Table created.
SQL> select count(*) from inner;
COUNT(*)
----------
531
SQL> select count(*) from outer;
COUNT(*)
----------
27774
SQL> set timing on
SQL> set autotrace traceonly explain
SQL> select * from outer where id in (select id from inner);
265 rows selected.
Elapsed: 00:00:00.03
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 MERGE JOIN
2 1 SORT (JOIN)
3 2 TABLE ACCESS (FULL) OF 'OUTER'
4 1 SORT (JOIN)
5 4 VIEW OF 'VW_NSO_1'
6 5 SORT (UNIQUE)
7 6 TABLE ACCESS (FULL) OF 'INNER'
SQL> select * from outer where exists (select 'x' from inner where inner.id=outer.id);
265 rows selected.
Elapsed: 00:00:08.02
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 FILTER
2 1 TABLE ACCESS (FULL) OF 'OUTER'
3 1 TABLE ACCESS (FULL) OF 'INNER'
没有对表分析之前:从上面看出(select id from inner)返回值比较少的话，用in比较快的；
第一句相当于select * from outer,(select distinct id from inner) where inner.id=outer.id
你可以试试，如果(select id from inner)返回值比(select id from outer)多的话，用exists快。
对表进行分析后，看看效果。使用了hash jion,两种方式效果相同了
SQL> analyze table inner compute statistics;
Table analyzed.
Elapsed: 00:00:00.06
SQL> analyze table outer compute statistics;
Table analyzed.
Elapsed: 00:00:00.08
SQL> select * from outer where id in (select id from inner);
265 rows selected.
Elapsed: 00:00:00.03
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=11 Card=531 Bytes=3717)
1 0 HASH JOIN (Cost=11 Card=531 Bytes=3717)
2 1 SORT (UNIQUE)
3 2 TABLE ACCESS (FULL) OF 'INNER' (Cost=2 Card=531 Bytes=1593)
4 1 TABLE ACCESS (FULL) OF 'OUTER' (Cost=6 Card=27774 Bytes=111096)
SQL> select * from outer where exists (select 'x' from inner where inner.id=outer.id);
265 rows selected.
Elapsed: 00:00:00.03
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=11 Card=531 Bytes=3717)
1 0 HASH JOIN (Cost=11 Card=531 Bytes=3717)
2 1 SORT (UNIQUE)
3 2 TABLE ACCESS (FULL) OF 'INNER' (Cost=2 Card=531 Bytes=1593)
4 1 TABLE ACCESS (FULL) OF 'OUTER' (Cost=6 Card=27774 Bytes=111096)
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