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：

--10gdrop 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; --12cdrop 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。

--10gSQL> 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 1kk 1000000 SQL> --12cSQL> 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：

--10gSQL> 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'); COUNT(*)---------- 0 SQL> select* from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST')); PLAN_TABLE_OUTPUT----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------SQL_ID 22t5mb43w55pr, child number 0-------------------------------------select count(*) from t1,t2 where t1.id=t2.id and t1.dep_id='kk' and notexists (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> --12cSQL> 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'); COUNT(*)---------- 1000000 SQL> select* from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST')); PLAN_TABLE_OUTPUT------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------SQL_ID 22t5mb43w55pr, child number 0-------------------------------------select count(*) from t1,t2 wheret1.id=t2.id and t1.dep_id='kk' and not exists (select 1 from t1,t2where 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

--10gSQL> 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。

--12cSQL> select count(*) from t1,t2 where t1.id=t2.id and t2.dep_id='kk'; COUNT(*)---------- 1000000 SQL> set line 1000SQL> set pages 1000SQL> col PLAN_TABLE_OUTPUT for a250SQL>SQL>SQL> 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='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 count(*) from t1,t2 wheret1.id=t2.id and t1.dep_id='kk' and not exists (select 1 from t1,t2where 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.

总结

以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作能带来一定的帮助，如果有疑问大家可以留言交流。