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详解oracle中not exists对外层查询的影响

来源：爱站网时间：2019-11-03编辑：网友分享
oracle中not exists对外层查询的影响大家了解吗？今天爱站技术频道小编就为大家举例详细解说一下，对这方面知识感兴趣的朋友可以随着爱站技术频道小编的步伐来了解一下吧！

oracle中not exists对外层查询的影响大家了解吗？今天爱站技术频道小编就为大家举例详细解说一下，对这方面知识感兴趣的朋友可以随着爱站技术频道小编的步伐来了解一下吧！

前言

最近同事发现了一个问题，在12c中跑的buffer get很高，但是在10g中跑的buffer很低。怀疑是不是12c的优化器有问题。

这个10g的环境和12c的环境，数据量大致一样，只是有很少部分的不同，但是就是这个很少部分不同，造成了not exists中的子查询返回不同的值，进而对外层查询产生不同的影响。

我们来用如下的代码模拟一下。

初始化数据：

--10g
drop table t1;
drop table t2;
 
create table t1 (id number,name varchar2(20),dep_id varchar2(10));
create table t2 (id number,name varchar2(20),dep_id varchar2(10));
 
insert into t1 select rownum,'a','kk' from dual connect by level <=3000000;
insert into t2 select rownum,'a','kk' from dual connect by level <=1000000;
insert into t2 select rownum,'a','mm' from dual;
 
commit;
 
 
--12c
drop table t1;
drop table t2;
 
create table t1 (id number,name varchar2(20),dep_id varchar2(10));
create table t2 (id number,name varchar2(20),dep_id varchar2(10));
 
 
insert into t1 select rownum,'a','kk' from dual connect by level <=3000000;
insert into t2 select rownum,'a','kk' from dual connect by level <=1000000;
 
commit;

我们看到，12c的数据和10g只是有很少的差别，t1表12c和10g都一样，t2表在12c只是少了一行数据。

--10g
SQL> select dep_id,count(*) from t1 group by dep_id;
 
DEP_ID     COUNT(*)
-------------------- ----------
kk      3000000
 
SQL> select dep_id,count(*) from t2 group by dep_id;
 
DEP_ID     COUNT(*)
-------------------- ----------
mm       1
kk      1000000
 
SQL>
 
 
--12c
SQL> select dep_id,count(*) from t1 group by dep_id;
 
DEP_ID     COUNT(*)
-------------------- ----------
kk      3000000
 
SQL> select dep_id,count(*) from t2 group by dep_id;
 
DEP_ID     COUNT(*)
-------------------- ----------
kk      1000000
 
SQL>

我们将要执行的sql语句是：

select count(*)
 from t1, t2
 where t1.id = t2.id
 and t1.dep_id = 'kk'
 and not exists (select 1
   from t1, t2
   where t1.id = t2.id
   and t2.dep_id = 'mm');

我们先来看执行情况的差距，10g的bufferget小，12c多：

--10g
SQL> select /*+ gather_plan_statistics */ count(*) from t1,t2 where t1.id=t2.id and t1.dep_id='kk' and not exists (select 1 from t1,t2 where t1.id=t2.id and t2.dep_id='mm');
 
 COUNT(*)
----------
   0
 
SQL> select* from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
 
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 22t5mb43w55pr, child number 0
-------------------------------------
select /*+ gather_plan_statistics */ count(*) from t1,t2 where t1.id=t2.id and t1.dep_id='kk' and not
exists (select 1 from t1,t2 where t1.id=t2.id and t2.dep_id='mm')
 
Plan hash value: 3404612428
 
------------------------------------------------------------------------------------------------------------------
| Id | Operation   | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT  |  |  1 |  |  1 |00:00:00.02 | 2086 |  |  |   |
| 1 | SORT AGGREGATE  |  |  1 |  1 |  1 |00:00:00.02 | 2086 |  |  |   |
|* 2 | FILTER    |  |  1 |  |  0 |00:00:00.02 | 2086 |  |  |   |
|* 3 | HASH JOIN   |  |  0 | 901K|  0 |00:00:00.01 |  0 | 39M| 5518K|   |
| 4 |  TABLE ACCESS FULL| T2 |  0 | 901K|  0 |00:00:00.01 |  0 |  |  |   |
|* 5 |  TABLE ACCESS FULL| T1 |  0 | 2555K|  0 |00:00:00.01 |  0 |  |  |   |
|* 6 | HASH JOIN   |  |  1 |  23 |  1 |00:00:00.02 | 2086 | 1517K| 1517K| 612K (0)|
|* 7 |  TABLE ACCESS FULL| T2 |  1 |  23 |  1 |00:00:00.02 | 2082 |  |  |   |
| 8 |  TABLE ACCESS FULL| T1 |  1 | 2555K|  1 |00:00:00.01 |  4 |  |  |   |
------------------------------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
 2 - filter( IS NULL)
 3 - access("T1"."ID"="T2"."ID")
 5 - filter("T1"."DEP_ID"='kk')
 6 - access("T1"."ID"="T2"."ID")
 7 - filter("T2"."DEP_ID"='mm')
 
Note
-----
 - dynamic sampling used for this statement
 
 
34 rows selected.
 
SQL>
 
 
--12c
SQL> select /*+ gather_plan_statistics */ count(*) from t1,t2 where t1.id=t2.id and t1.dep_id='kk' and not exists (select 1 from t1,t2 where t1.id=t2.id and t2.dep_id='mm');
 
 COUNT(*)
----------
 1000000
 
SQL> select* from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
 
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 22t5mb43w55pr, child number 0
-------------------------------------
select /*+ gather_plan_statistics */ count(*) from t1,t2 where
t1.id=t2.id and t1.dep_id='kk' and not exists (select 1 from t1,t2
where t1.id=t2.id and t2.dep_id='mm')
 
Plan hash value: 1692274438
 
--------------------------------------------------------------------------------------------------------------------
| Id | Operation    | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT  |  |  1 |  |  1 |00:00:00.79 | 10662 |  | |  |
| 1 | SORT AGGREGATE  |  |  1 |  1 |  1 |00:00:00.79 | 10662 |  | |  |
|* 2 | FILTER    |  |  1 |  | 1000K|00:00:00.74 | 10662 |  | |  |
|* 3 | HASH JOIN   |  |  1 | 1215K| 1000K|00:00:00.52 | 8579 | 43M| 6111K| 42M (0)|
| 4 |  TABLE ACCESS FULL | T2 |  1 | 1215K| 1000K|00:00:00.01 | 2083 |  | |  |
|* 5 |  TABLE ACCESS FULL | T1 |  1 | 2738K| 3000K|00:00:00.07 | 6496 |  | |  |
|* 6 | HASH JOIN RIGHT SEMI|  |  1 |  35 |  0 |00:00:00.02 | 2083 | 1245K| 1245K| 461K (0)|
|* 7 |  TABLE ACCESS FULL | T2 |  1 |  23 |  0 |00:00:00.02 | 2083 |  | |  |
| 8 |  TABLE ACCESS FULL | T1 |  0 | 2738K|  0 |00:00:00.01 |  0 |  | |  |
--------------------------------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
 2 - filter( IS NULL)
 3 - access("T1"."ID"="T2"."ID")
 5 - filter("T1"."DEP_ID"='kk')
 6 - access("T1"."ID"="T2"."ID")
 7 - filter("T2"."DEP_ID"='mm')
 
Note
-----
 - dynamic statistics used: dynamic sampling (level=2)
 
 
35 rows selected.
 
SQL>
SQL>

可以看到第23，24行，在10g中运行时，buffers是0，而在12c中，即78，79行，buffer是2083+6496。

也就是说在10g中，外层查询不进行t1和t2的扫描，直接返回结果了，而在12c中，外层查询还要进行t1表和t2表层扫描才返回结果。

这其实不是10g和12c的差别，而是not exists的返回数据对外层的影响。子查询要返回0行记录，才满足not exist的条件，从而返回外层查询结果。

在10g中，子查询返回了一行记录

--10g
SQL> select 1 from t1,t2 where t1.id=t2.id and t2.dep_id='mm';
 
   1
----------
   1
 
SQL>

不满足not exists（即0行才满足），所以，也就不用在外层继续查询了。直接返回记录0行。

在12c中，子查询返回0行记录，满足not exist的条件，所以还需要在外层查询中继续查询。

--12c
SQL> select count(*) from t1,t2 where t1.id=t2.id and t2.dep_id='kk';
 
 COUNT(*)
----------
 1000000
 
SQL> set line 1000
SQL> set pages 1000
SQL> col PLAN_TABLE_OUTPUT for a250
SQL>
SQL>
SQL> select /*+ gather_plan_statistics */ count(*) from t1,t2 where t1.id=t2.id and t1.dep_id='kk' and not exists (select 1 from t1,t2 where t1.id=t2.id and t2.dep_id='kk');
 
 COUNT(*)
----------
   0
 
SQL> select* from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
 
PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID c5hj2p2jt1fxf, child number 0
-------------------------------------
select /*+ gather_plan_statistics */ count(*) from t1,t2 where
t1.id=t2.id and t1.dep_id='kk' and not exists (select 1 from t1,t2
where t1.id=t2.id and t2.dep_id='kk')
 
Plan hash value: 1692274438
 
--------------------------------------------------------------------------------------------------------------------
| Id | Operation    | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT  |  |  1 |  |  1 |00:00:00.28 | 2087 |  | |  |
| 1 | SORT AGGREGATE  |  |  1 |  1 |  1 |00:00:00.28 | 2087 |  | |  |
|* 2 | FILTER    |  |  1 |  |  0 |00:00:00.28 | 2087 |  | |  |
|* 3 | HASH JOIN   |  |  0 | 1215K|  0 |00:00:00.01 |  0 | 69M| 7428K|   |
| 4 |  TABLE ACCESS FULL | T2 |  0 | 1215K|  0 |00:00:00.01 |  0 |  | |  |
|* 5 |  TABLE ACCESS FULL | T1 |  0 | 2738K|  0 |00:00:00.01 |  0 |  | |  |
|* 6 | HASH JOIN RIGHT SEMI|  |  1 | 2738K|  1 |00:00:00.28 | 2087 | 43M| 6111K| 42M (0)|
|* 7 |  TABLE ACCESS FULL | T2 |  1 | 1215K| 1000K|00:00:00.12 | 2083 |  | |  |
| 8 |  TABLE ACCESS FULL | T1 |  1 | 2738K|  1 |00:00:00.01 |  4 |  | |  |
--------------------------------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
 2 - filter( IS NULL)
 3 - access("T1"."ID"="T2"."ID")
 5 - filter("T1"."DEP_ID"='kk')
 6 - access("T1"."ID"="T2"."ID")
 7 - filter("T2"."DEP_ID"='kk')
 
Note
-----
 - dynamic statistics used: dynamic sampling (level=2)
 
 
35 rows selected.
 
SQL>

可以看到第38，39行的buffer为0.

上面是介绍，大家都熟悉了吗？今天爱站技术频道小编就和大家分享到这里了，想要学习更多的专业知识，请收藏js.aizhan.com吧！

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