oracle中not exists对外层查询的影响详解

作者：OracleBlog 字体：[增加减小] 来源：互联网时间：2017-05-11

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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;

</div>

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>

</div>

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');

</div>

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>

</div>

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>

</div>

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';

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