[20180808]exists and not exists.txt

--//生產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;
Table created.

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
--------------------
T1

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

-------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id? | Operation??????????? | Name | Starts | E-Rows |E-Bytes|E-Temp | Cost (%CPU)| E-Time?? | A-Rows |?? A-Time?? | Buffers | Reads? |? OMem |? 1Mem | Used-Mem |
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
|?? 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.
--//實際上對于當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' );
OBJECT_NAME
--------------------
T1

SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
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
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id? | Operation??????????????????????????? | Name????? | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time?? | A-Rows |?? A-Time?? | Buffers | Reads? |? OMem |? 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|?? 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 |?????? |?????? |????????? |
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
?? 1 - SEL$5DA710D3
?? 2 - SEL$5DA710D3 / T2@SEL$2
?? 3 - SEL$5DA710D3 / T2@SEL$2
?? 4 - SEL$5DA710D3 / T1@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
?? 1 - access("T2"."OBJECT_ID"="T1"."OBJECT_ID")
?? 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);

SCOTT@test01p> select object_name? from t1 where? exists (select 1 from t2 where t2.object_id=t1.object_id and t2.flag='0' );
OBJECT_NAME
--------------------
T1

SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
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: 4193600567
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id? | Operation????????????????????????????? | Name????? | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time?? | A-Rows |?? A-Time?? | Buffers | Reads? |? OMem |? 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|?? 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 |?????? |?????? |????????? |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
?? 4 - SEL$5DA710D3 / T2@SEL$2
?? 5 - SEL$5DA710D3 / T2@SEL$2
?? 6 - SEL$5DA710D3 / T1@SEL$1
?? 7 - SEL$5DA710D3 / T1@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
?? 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...

轉載于:https://www.cnblogs.com/lfree/p/9445628.html

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