[20180808]exists and not exists.txt

--//生産系統遇到的一個性能問題,通過例子來說明:

1.環境:

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

SCOTT@test01p> create table t1 as select * from all_objects;

Table created.

SCOTT@test01p> create table t2 as select object_id,'1' flag from t1;

SCOTT@test01p> select max(object_id) from t2;

MAX(OBJECT_ID)

--------------

107828

SCOTT@test01p> update t2 set flag='0' where object_id=107828;

1 row updated.

SCOTT@test01p> commit ;

Commit complete.

SCOTT@test01p> create index i_t2_flag on t2(flag);

Index created.

--//分析表,并且t2的flag字段建立直方圖.

execute sys.dbms_stats.gather_table_stats ( OwnName => user,TabName => 't1',Estimate_Percent => NULL,Method_Opt => 'FOR ALL COLUMNS SIZE 1 ',Cascade => True ,No_Invalidate => false);

execute sys.dbms_stats.gather_table_stats ( OwnName => user,TabName => 't2',Estimate_Percent => NULL,Method_Opt => 'FOR ALL COLUMNS SIZE 1 for columns flag size 10 ',Cascade => True ,No_Invalidate => false);

2.測試:

SCOTT@test01p> alter session set statistics_level=all;

Session altered.

SCOTT@test01p> select object_name from t1 where not exists (select 1 from t2 where t2.object_id=t1.object_id and t2.flag='1' );

OBJECT_NAME

--------------------

SCOTT@test01p> @ dpc '' ''

PLAN_TABLE_OUTPUT

-------------------------------------

SQL_ID d4qcxhmwy49r1, child number 0

select object_name from t1 where not exists (select 1 from t2 where

t2.object_id=t1.object_id and t2.flag='1' )

Plan hash value: 629543484

-------------------------------------------------------------------------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | | | | 728 (100)| | 1 |00:00:00.24 | 1667 | 1511 | | | |

|* 1 | HASH JOIN RIGHT ANTI| | 1 | 899 | 33263 | 1672K| 728 (1)| 00:00:01 | 1 |00:00:00.24 | 1667 | 1511 | 5536K| 3056K| 5658K (0)|

|* 2 | TABLE ACCESS FULL | T2 | 1 | 89876 | 614K| | 46 (3)| 00:00:01 | 89876 |00:00:00.02 | 152 | 0 | | | |

| 3 | TABLE ACCESS FULL | T1 | 1 | 89877 | 2633K| | 421 (1)| 00:00:01 | 89877 |00:00:00.11 | 1515 | 1511 | | | |

Query Block Name / Object Alias (identified by operation id):

-------------------------------------------------------------

1 - SEL$5DA710D3

2 - SEL$5DA710D3 / T2@SEL$2

3 - SEL$5DA710D3 / T1@SEL$1

Predicate Information (identified by operation id):

---------------------------------------------------

1 - access("T2"."OBJECT_ID"="T1"."OBJECT_ID")

2 - filter("T2"."FLAG"='1')

--//仔細看id-2.過濾條件是 2 - filter("T2"."FLAG"='1').這樣即使你建立索引在t2.flag也不會使用.因為flag='1'占大多數.

--//實際上對于目前應用改成如下是等效的.因為flag僅僅兩種取值'0','1'.

SCOTT@test01p> select object_name from t1 where exists (select 1 from t2 where t2.object_id=t1.object_id and t2.flag='0' );

SQL_ID 1y5xvtwz0u11f, child number 0

select object_name from t1 where exists (select 1 from t2 where

t2.object_id=t1.object_id and t2.flag='0' )

Plan hash value: 1273788863

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | | | 423 (100)| | 1 |00:00:00.19 | 1518 | 1512 | | | |

|* 1 | HASH JOIN RIGHT SEMI | | 1 | 1 | 37 | 423 (1)| 00:00:01 | 1 |00:00:00.19 | 1518 | 1512 | 2168K| 2168K| 697K (0)|

| 2 | TABLE ACCESS BY INDEX ROWID BATCHED| T2 | 1 | 1 | 7 | 2 (0)| 00:00:01 | 1 |00:00:00.04 | 3 | 1 | | | |

|* 3 | INDEX RANGE SCAN | I_T2_FLAG | 1 | 1 | | 1 (0)| 00:00:01 | 1 |00:00:00.04 | 2 | 1 | | | |

| 4 | TABLE ACCESS FULL | T1 | 1 | 89877 | 2633K| 421 (1)| 00:00:01 | 89877 |00:00:00.12 | 1515 | 1511 | | | |

3 - SEL$5DA710D3 / T2@SEL$2

4 - SEL$5DA710D3 / T1@SEL$1

3 - access("T2"."FLAG"='0')

--//實際上到具體應用object_id字段是主鍵,如果在上面建立索引,邏輯讀更小.

CREATE UNIQUE INDEX SCOTT.pk_t1 ON SCOTT.T1 (OBJECT_ID);

ALTER TABLE SCOTT.T1 ADD CONSTRAINT pk_t1 PRIMARY KEY (OBJECT_ID);

CREATE UNIQUE INDEX SCOTT.pk_t2 ON SCOTT.T2 (OBJECT_ID);

ALTER TABLE SCOTT.T2 ADD CONSTRAINT pk_t2 PRIMARY KEY (OBJECT_ID);

Plan hash value: 4193600567

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | | | 3 (100)| | 1 |00:00:00.04 | 6 | 2 | | | |

| 1 | NESTED LOOPS | | 1 | | | | | 1 |00:00:00.04 | 6 | 2 | | | |

| 2 | NESTED LOOPS | | 1 | 1 | 37 | 3 (0)| 00:00:01 | 1 |00:00:00.03 | 5 | 1 | | | |

| 3 | SORT UNIQUE | | 1 | 1 | 7 | 2 (0)| 00:00:01 | 1 |00:00:00.01 | 3 | 0 | 2048 | 2048 | 2048 (0)|

| 4 | TABLE ACCESS BY INDEX ROWID BATCHED| T2 | 1 | 1 | 7 | 2 (0)| 00:00:01 | 1 |00:00:00.01 | 3 | 0 | | | |

|* 5 | INDEX RANGE SCAN | I_T2_FLAG | 1 | 1 | | 1 (0)| 00:00:01 | 1 |00:00:00.01 | 2 | 0 | | | |

|* 6 | INDEX UNIQUE SCAN | PK_T1 | 1 | 1 | | 0 (0)| | 1 |00:00:00.03 | 2 | 1 | | | |

| 7 | TABLE ACCESS BY INDEX ROWID | T1 | 1 | 1 | 30 | 1 (0)| 00:00:01 | 1 |00:00:00.01 | 1 | 1 | | | |

4 - SEL$5DA710D3 / T2@SEL$2

5 - SEL$5DA710D3 / T2@SEL$2

6 - SEL$5DA710D3 / T1@SEL$1

7 - SEL$5DA710D3 / T1@SEL$1

5 - access("T2"."FLAG"='0')

6 - access("T2"."OBJECT_ID"="T1"."OBJECT_ID")

Note

-----

- this is an adaptive plan

--//而select object_name from t1 where not exists (select 1 from t2 where t2.object_id=t1.object_id and t2.flag='1' );執行計劃不變.不再貼出.

--//我有時候想開發寫sql代碼過腦子沒有,有時候真的很無語很無奈...