[20180928]exists与cardinality.txt

--//优化遇到的问题,做一个例子演示出来.
1.环境:

SCOTT@test01p> @　ver1
PORT_STRING                    VERSION        BANNER                                                                               CON_ID
------------------------------ -------------- -------------------------------------------------------------------------------- ----------
IBMPC/WIN_NT64-9.1.0           12.1.0.1.0     Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production              0

2.测试建立:
create table t1 as select rownum id ,lpad('a',100,'a') vc from dual connect by level<=1000;
create table t2 as select rownum idx,mod(rownum,1000)+1 id ,lpad('b',20,'b') vc from dual connect by level<=40000;

create unique index pk_t1 on t1(id);
alter table t1  add constraint pk_t1  primary key (id);

create unique index pk_t2 on t2(idx);
alter table t2  add constraint pk_t2  primary key (idx);

create index i_t2_id on t2(id);

3.测试1:

SCOTT@test01p> alter session set statistics_level = all;
Session altered.

SCOTT@test01p> select * from t1 where exists (select 1 from t2 where t2.id=t1.id) and t1.id=32;
        ID VC
---------- ----------------------------------------------------------------------------------------------------
        32 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa


--//执行计划如下:
SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  ajnkhp6968v8r, child number 1
-------------------------------------
select * from t1 where exists (select 1 from t2 where t2.id=t1.id) and
t1.id=32
Plan hash value: 1277462125
-------------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name    | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers | Reads  |
-------------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |         |      1 |        |       |     3 (100)|          |      1 |00:00:00.02 |       5 |      1 |
|   1 |  NESTED LOOPS SEMI           |         |      1 |      1 |   109 |     3   (0)| 00:00:01 |      1 |00:00:00.02 |       5 |      1 |
|   2 |   TABLE ACCESS BY INDEX ROWID| T1      |      1 |      1 |   105 |     2   (0)| 00:00:01 |      1 |00:00:00.01 |       3 |      0 |
|*  3 |    INDEX UNIQUE SCAN         | PK_T1   |      1 |      1 |       |     1   (0)| 00:00:01 |      1 |00:00:00.01 |       2 |      0 |
|*  4 |   INDEX RANGE SCAN           | I_T2_ID |      1 |     40 |   160 |     1   (0)| 00:00:01 |      1 |00:00:00.02 |       2 |      1 |
-------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$5DA710D3
   2 - SEL$5DA710D3 / T1@SEL$1
   3 - SEL$5DA710D3 / T1@SEL$1
   4 - SEL$5DA710D3 / T2@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
   3 - access("T1"."ID"=32)
   4 - access("T2"."ID"=32)
       filter("T2"."ID"="T1"."ID")

--//实际上开始让我困惑的是id=4,E_rows=40,实际上exists只要1条满足条件就ok了.不需要继续判断,有点短路的作用.
--//这里非常容易误判,我们生产系统E_rows更高,差点给误导了.

4.测试2:
--//测试not exists的情况如下:
SCOTT@test01p> select * from t1 where not exists (select 1 from t2 where t2.id=t1.id) and t1.id=32;

no rows selected

SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  21f5mqdya13v8, child number 1
-------------------------------------
select * from t1 where not exists (select 1 from t2 where t2.id=t1.id)
and t1.id=32
Plan hash value: 1740670345
----------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name    | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers |
----------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |         |      1 |        |       |     3 (100)|          |      0 |00:00:00.01 |       5 |
|   1 |  NESTED LOOPS ANTI           |         |      1 |      1 |   109 |     3   (0)| 00:00:01 |      0 |00:00:00.01 |       5 |
|   2 |   TABLE ACCESS BY INDEX ROWID| T1      |      1 |      1 |   105 |     2   (0)| 00:00:01 |      1 |00:00:00.01 |       3 |
|*  3 |    INDEX UNIQUE SCAN         | PK_T1   |      1 |      1 |       |     1   (0)| 00:00:01 |      1 |00:00:00.01 |       2 |
|*  4 |   INDEX RANGE SCAN           | I_T2_ID |      1 |     40 |   160 |     1   (0)| 00:00:01 |      1 |00:00:00.01 |       2 |
----------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$5DA710D3
   2 - SEL$5DA710D3 / T1@SEL$1
   3 - SEL$5DA710D3 / T1@SEL$1
   4 - SEL$5DA710D3 / T2@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
   3 - access("T1"."ID"=32)
   4 - access("T2"."ID"=32)
31 rows selected.


--//实际上我遇到的优化问题就是一个项目表有2千多条记录,查询业务表有那些项目已经开展的.每次进入程序界面就
--//以列表的形式显示,调用如下:

select * from 项目表 where exists (select 1 from 业务表 where 业务表.项目_id=项目表.id);

--//业务表巨大无比,看到以上执行计划的E_rows达到上万,习惯思维,差点被误导.
--//我自己查看我工作笔记,这个在业务表上"项目_id"字段索引还是我去年建立的.该索引重复值太多,而且这个索引除了这样的查询一点用都没有.
--//有时候想开发为什么写这样的sql语句,写前考虑没有.代价太大了.
--//真心希望开发写sql语句想一想.....