[20200909]为什么执行计划不是出现hash join semi.txt
--//别人问的问题,为什么执行计划hash join semi,通过例子说明问题:
1.环境:
SCOTT@test01p> @ ver1
PORT_STRING VERSION BANNER CON_ID
------------------------------ -------------- -------------------------------------------------------------------------------- ----------
IBMPC/WIN_NT64-9.1.0 12.2.0.1.0 Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production 0
2.测试:
SCOTT@test01p> alter session set statistics_level = all;
Session altered.
SCOTT@test01p> select * from emp where exists (select 1 from dept where dname='SALES' and dept.deptno=emp.deptno);
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7499 ALLEN SALESMAN 7698 1981-02-20 00:00:00 1600 300 30
7521 WARD SALESMAN 7698 1981-02-22 00:00:00 1250 500 30
7654 MARTIN SALESMAN 7698 1981-09-28 00:00:00 1250 1400 30
7698 BLAKE MANAGER 7839 1981-05-01 00:00:00 2850 30
7844 TURNER SALESMAN 7698 1981-09-08 00:00:00 1500 0 30
7900 JAMES CLERK 7698 1981-12-03 00:00:00 950 30
6 rows selected.
SCOTT@test01p> @ dpc '' advanced
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 8053b6kh802ky, child number 0
-------------------------------------
select * from emp where exists (select 1 from dept where dname='SALES'
and dept.deptno=emp.deptno)
Plan hash value: 367693176
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 5 (100)| | 6 |00:00:00.01 | 10 | | | |
|* 1 | HASH JOIN | | 1 | 5 | 255 | 5 (0)| 00:00:01 | 6 |00:00:00.01 | 10 | 2545K| 2545K| 785K (0)|
| 2 | TABLE ACCESS BY INDEX ROWID BATCHED| DEPT | 1 | 1 | 13 | 2 (0)| 00:00:01 | 1 |00:00:00.01 | 2 | | | |
|* 3 | INDEX RANGE SCAN | I_DEPT_DNAME | 1 | 1 | | 1 (0)| 00:00:01 | 1 |00:00:00.01 | 1 | | | |
| 4 | TABLE ACCESS FULL | EMP | 1 | 14 | 532 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 8 | | | |
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$5DA710D3
2 - SEL$5DA710D3 / DEPT@SEL$2
3 - SEL$5DA710D3 / DEPT@SEL$2
4 - SEL$5DA710D3 / EMP@SEL$1
--//别人问为什么不出现HASH JOIN semi.而是hash join,是连接顺序的问题吗?因为这样dept是驱动表.
select /*+ leading(emp) */ * from emp where exists (select 1 from dept where dname='SALES' and dept.deptno=emp.deptno);
Plan hash value: 4254668763
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 5 (100)| | 6 |00:00:00.01 | 9 | | | |
|* 1 | HASH JOIN | | 1 | 5 | 255 | 5 (0)| 00:00:01 | 6 |00:00:00.01 | 9 | 1098K| 1098K| 664K (0)|
| 2 | TABLE ACCESS FULL | EMP | 1 | 14 | 532 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 7 | | | |
| 3 | TABLE ACCESS BY INDEX ROWID BATCHED| DEPT | 1 | 1 | 13 | 2 (0)| 00:00:01 | 1 |00:00:00.01 | 2 | | | |
|* 4 | INDEX RANGE SCAN | I_DEPT_DNAME | 1 | 1 | | 1 (0)| 00:00:01 | 1 |00:00:00.01 | 1 | | | |
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
--//一样没有出现HASH JOIN SEMI连接.因为dept.deptno是主键吗?
3.继续:
SCOTT@test01p> create table deptx as select * from dept ;
Table created.
select * from emp where exists (select 1 from deptx dept where dname='SALES' and dept.deptno=emp.deptno);
SCOTT@test01p> @ dpc '' advanced
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 5scp0hawkubrw, child number 0
-------------------------------------
select * from emp where exists (select 1 from deptx dept where
dname='SALES' and dept.deptno=emp.deptno)
Plan hash value: 1460795715
-------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 6 (100)| | 6 |00:00:00.01 | 17 | | | |
|* 1 | HASH JOIN SEMI | | 1 | 5 | 255 | 6 (0)| 00:00:01 | 6 |00:00:00.01 | 17 | 1098K| 1098K| 669K (0)|
| 2 | TABLE ACCESS FULL| EMP | 1 | 14 | 532 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 7 | | | |
|* 3 | TABLE ACCESS FULL| DEPTX | 1 | 1 | 13 | 3 (0)| 00:00:01 | 1 |00:00:00.01 | 10 | | | |
-------------------------------------------------------------------------------------------------------------------------------------------------
--//这次出现HASH JOIN SEMI.
SCOTT@test01p> create index i_deptx_dname on deptx(dname);
Index created.
select * from emp where exists (select 1 from deptx dept where dname='SALES' and dept.deptno=emp.deptno);
Plan hash value: 2867647663
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 5 (100)| | 6 |00:00:00.01 | 9 | | | |
|* 1 | HASH JOIN SEMI | | 1 | 5 | 255 | 5 (0)| 00:00:01 | 6 |00:00:00.01 | 9 | 1098K| 1098K| 691K (0)|
| 2 | TABLE ACCESS FULL | EMP | 1 | 14 | 532 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 7 | | | |
| 3 | TABLE ACCESS BY INDEX ROWID BATCHED| DEPTX | 1 | 1 | 13 | 2 (0)| 00:00:01 | 1 |00:00:00.01 | 2 | | | |
|* 4 | INDEX RANGE SCAN | I_DEPTX_DNAME | 1 | 1 | | 1 (0)| 00:00:01 | 1 |00:00:00.01 | 1 | | | |
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
--//这次也出现HASH JOIN SEMI.如果将dept作为驱动表呢?
select /*+ leading(dept@sub ) */ * from emp where exists (select /*+ qb_name(sub) */ 1 from deptx dept where dname='SALES' and dept.deptno=emp.deptno);
Plan hash value: 743985058
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 6 (100)| | 6 |00:00:00.01 | 46 | | | |
|* 1 | HASH JOIN | | 1 | 5 | 255 | 6 (17)| 00:00:01 | 6 |00:00:00.01 | 46 | 2545K| 2545K| 675K (0)|
| 2 | SORT UNIQUE | | 1 | 1 | 13 | 2 (0)| 00:00:01 | 1 |00:00:00.01 | 2 | 2048 | 2048 | 2048 (0)|
| 3 | TABLE ACCESS BY INDEX ROWID BATCHED| DEPTX | 1 | 1 | 13 | 2 (0)| 00:00:01 | 1 |00:00:00.01 | 2 | | | |
|* 4 | INDEX RANGE SCAN | I_DEPTX_DNAME | 1 | 1 | | 1 (0)| 00:00:01 | 1 |00:00:00.01 | 1 | | | |
| 5 | TABLE ACCESS FULL | EMP | 1 | 14 | 532 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 8 | | | |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
--//可以发现这种情况,只要子查询展开后,dept作为驱动表,Operation就不会出现HASH JOIN SEMI.
--//前面没有出现hash join semi主要原因是dept.deptno是主键或者唯一索引.
SCOTT@test01p> create unique index i_deptx_deptno on deptx(deptno);
Index created.
select * from emp where exists (select 1 from deptx dept where dname='SALES' and dept.deptno=emp.deptno);
Plan hash value: 2939079003
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 5 (100)| | 6 |00:00:00.01 | 10 | | | |
|* 1 | HASH JOIN | | 1 | 5 | 255 | 5 (0)| 00:00:01 | 6 |00:00:00.01 | 10 | 2545K| 2545K| 710K (0)|
| 2 | TABLE ACCESS BY INDEX ROWID BATCHED| DEPTX | 1 | 1 | 13 | 2 (0)| 00:00:01 | 1 |00:00:00.01 | 2 | | | |
|* 3 | INDEX RANGE SCAN | I_DEPTX_DNAME | 1 | 1 | | 1 (0)| 00:00:01 | 1 |00:00:00.01 | 1 | | | |
| 4 | TABLE ACCESS FULL | EMP | 1 | 14 | 532 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 8 | | | |
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------
--//这样就不出现HASH JOIN SEMI.
--//平时自己也很少注意这些细节问题...
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