push_subq提示

PUSH_SUBQ 可以用来控制子查询的执行 这个是PUSH_SUBQ 的本意

我那个例子的意思是说:

PUSH_SUBQ 本质上是个CBO的hints(当然RBO也提不上hints)

由于PUSH_SUBQ 的引入就是为了来解决unnesting的某些不足

所以在不同的版本上,这个hints发挥的作用也有所不同了.

 

8i上这个提示的作用更接近本原:

 

$ sqlplus "/ as sysdba"

SQL*Plus: Release 8.1.5.0.0 - Production on Sun Sep 12 20:51:21 2004

(c) Copyright 1999 Oracle Corporation.  All rights reserved.

Connected to:

Oracle8i Enterprise Edition Release 8.1.5.0.0 - Production

With the Partitioning and Java options

PL/SQL Release 8.1.5.0.0 - Production

 

SQL> connect scott/tiger

Connected.

SQL> create table dept1 as select * from dept;

Table created.

 

SQL> set linesize 120

SQL> select a.*     

  2  from emp a,dept b where a.deptno = b.deptno and exists (select 1 from dept1 where dept1.dname = b.dname);

 

     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO

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

      7369 SMITH      CLERK           7902 17-DEC-80        800                    20

      7499 ALLEN      SALESMAN        7698 20-FEB-81       1600        300         30

      7521 WARD       SALESMAN        7698 22-FEB-81       1250        500         30

      7566 JONES      MANAGER         7839 02-APR-81       2975                    20

      7654 MARTIN     SALESMAN        7698 28-SEP-81       1250       1400         30

      7698 BLAKE      MANAGER         7839 01-MAY-81       2850                    30

      7782 CLARK      MANAGER         7839 09-JUN-81       2450                    10

      7788 SCOTT      ANALYST         7566 19-APR-87       3000                    20

      7839 KING       PRESIDENT            17-NOV-81       5000                    10

      7844 TURNER     SALESMAN        7698 08-SEP-81       1500          0         30

      7876 ADAMS      CLERK           7788 23-MAY-87       1100                    20

 

     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO

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

      7900 JAMES      CLERK           7698 03-DEC-81        950                    30

      7902 FORD       ANALYST         7566 03-DEC-81       3000                    20

      7934 MILLER     CLERK           7782 23-JAN-82       1300                    10

 

14 rows selected.

 

 

Execution Plan

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

   0      SELECT STATEMENT ptimizer=CHOOSE

   1    0   FILTER ---------------'最初push_subq的使命是为了消除/提高这个filter的效率的'

   2    1     NESTED LOOPS

   3    2       TABLE ACCESS (FULL) OF 'EMP'

   4    2       TABLE ACCESS (BY INDEX ROWID) OF 'DEPT'

   5    4         INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)

   6    1     TABLE ACCESS (FULL) OF 'DEPT1'

 

 

Statistics

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

          0  recursive calls

         52  db block gets

         21  consistent gets

          0  physical reads

          0  redo size

       2715  bytes sent via SQL*Net to client

        751  bytes received via SQL*Net from client

          4  SQL*Net roundtrips to/from client

          1  sorts (memory)

          0  sorts (disk)

         14  rows processed

 

 

用于push_subq是个CBO hints，这里我们可以看到COST的出现:

 

 

SQL> select /*+ push_subq */ a.*

  2  from emp a,dept b where a.deptno = b.deptno and exists (select 1 from dept1 where dept1.dname = b.dname);

 

     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO

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

      7782 CLARK      MANAGER         7839 09-JUN-81       2450                    10

      7839 KING       PRESIDENT            17-NOV-81       5000                    10

      7934 MILLER     CLERK           7782 23-JAN-82       1300                    10

      7369 SMITH      CLERK           7902 17-DEC-80        800                    20

      7566 JONES      MANAGER         7839 02-APR-81       2975                    20

      7788 SCOTT      ANALYST         7566 19-APR-87       3000                    20

      7876 ADAMS      CLERK           7788 23-MAY-87       1100                    20

      7902 FORD       ANALYST         7566 03-DEC-81       3000                    20

      7499 ALLEN      SALESMAN        7698 20-FEB-81       1600        300         30

      7521 WARD       SALESMAN        7698 22-FEB-81       1250        500         30

      7654 MARTIN     SALESMAN        7698 28-SEP-81       1250       1400         30

 

     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO

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

      7698 BLAKE      MANAGER         7839 01-MAY-81       2850                    30

      7844 TURNER     SALESMAN        7698 08-SEP-81       1500          0         30

      7900 JAMES      CLERK           7698 03-DEC-81        950                    30

 

14 rows selected.

 

 

Execution Plan

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

   0      SELECT STATEMENT ptimizer=CHOOSE (Cost=3 Card=1 Bytes=122)

   1    0   NESTED LOOPS (Cost=3 Card=1 Bytes=122)

   2    1     TABLE ACCESS (FULL) OF 'DEPT' (Cost=1 Card=2 Bytes=44)

   3    2       TABLE ACCESS (FULL) OF 'DEPT1' (Cost=1 Card=1 Bytes=9)--------'这里消除了之前的filter'

   4    1     TABLE ACCESS (FULL) OF 'EMP' (Cost=1 Card=21 Bytes=2100)

 

 

Statistics

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

        204  recursive calls

         85  db block gets

         38  consistent gets

          0  physical reads

          0  redo size

       2706  bytes sent via SQL*Net to client

        768  bytes received via SQL*Net from client

          4  SQL*Net roundtrips to/from client

          5  sorts (memory)

          0  sorts (disk)

         14  rows processed

 

再看CBO下:

 

正常情况下,没什么好说的:

 

 

SQL> set autotrace traceonly

SQL> exec dbms_stats.gather_schema_stats('SCOTT')

 

PL/SQL procedure successfully completed.

 

SQL> select a.*

  2  from emp a,dept b where a.deptno = b.deptno and exists (select 1 from dept1 where dept1.dname = b.dname);

 

14 rows selected.

 

 

Execution Plan

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

   0      SELECT STATEMENT ptimizer=CHOOSE (Cost=2 Card=5 Bytes=275)

   1    0   FILTER

   2    1     NESTED LOOPS (Cost=2 Card=5 Bytes=275)

   3    2       TABLE ACCESS (FULL) OF 'DEPT' (Cost=1 Card=1 Bytes=18)

   4    2       TABLE ACCESS (FULL) OF 'EMP' (Cost=1 Card=14 Bytes=518)

   5    1     TABLE ACCESS (FULL) OF 'DEPT1' (Cost=1 Card=1 Bytes=9)

 

 

Statistics

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

         39  recursive calls

         68  db block gets

         21  consistent gets

          0  physical reads

          0  redo size

       2708  bytes sent via SQL*Net to client

        751  bytes received via SQL*Net from client

          4  SQL*Net roundtrips to/from client

          1  sorts (memory)

          0  sorts (disk)

         14  rows processed

 

在CBO下push_subq发挥了同样的作用:

 

SQL> select /*+ push_subq */ a.*

  2  from emp a,dept b where a.deptno = b.deptno and exists (select 1 from dept1 where dept1.dname = b.dname);

 

14 rows selected.

 

Execution Plan

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

   0      SELECT STATEMENT ptimizer=CHOOSE (Cost=2 Card=5 Bytes=275)

   1    0   NESTED LOOPS (Cost=2 Card=5 Bytes=275)

   2    1     TABLE ACCESS (FULL) OF 'DEPT' (Cost=1 Card=1 Bytes=18)

   3    2       TABLE ACCESS (FULL) OF 'DEPT1' (Cost=1 Card=1 Bytes=9)---'注意这里'

   4    1     TABLE ACCESS (FULL) OF 'EMP' (Cost=1 Card=14 Bytes=518)

 

Statistics

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

          0  recursive calls

         84  db block gets

         11  consistent gets

          0  physical reads

          0  redo size

       2709  bytes sent via SQL*Net to client

        768  bytes received via SQL*Net from client

          4  SQL*Net roundtrips to/from client

          1  sorts (memory)

          0  sorts (disk)

         14  rows processed

 

而在Oracle9i之中:

 

[oracle@jumper oracle]$ sqlplus "/ as sysdba"

SQL*Plus: Release 9.2.0.3.0 - Production on Sun Sep 12 21:42:57 2004

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Connected to:

Oracle9i Enterprise Edition Release 9.2.0.3.0 - Production

With the Partitioning, OLAP and Oracle Data Mining options

JServer Release 9.2.0.3.0 - Production

 

SQL> connect scott/tiger

Connected.

SQL> set linesize 120

SQL> set autotrace traceonly

SQL> select a.*

  2  from emp a,dept b where a.deptno = b.deptno and exists (select 1 from dept1 where dept1.dname = b.dname);

11 rows selected.

 

Execution Plan

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

   0      SELECT STATEMENT ptimizer=CHOOSE

   1    0   FILTER

   2    1     NESTED LOOPS

   3    2       TABLE ACCESS (FULL) OF 'EMP'

   4    2       TABLE ACCESS (BY INDEX ROWID) OF 'DEPT'

   5    4         INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)

   6    1     TABLE ACCESS (FULL) OF 'DEPT1'

 

Statistics

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

          0  recursive calls

          0  db block gets

         26  consistent gets

          0  physical reads

          0  redo size

       1164  bytes sent via SQL*Net to client

        503  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

         11  rows processed

 

SQL> select /*+ push_subq */ a.*

  2  from emp a,dept b where a.deptno = b.deptno and exists (select 1 from dept1 where dept1.dname = b.dname);

 

11 rows selected.

 

Execution Plan

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

   0      SELECT STATEMENT ptimizer=CHOOSE (Cost=8 Card=82 Bytes=9676)

   1    0   HASH JOIN (SEMI) (Cost=8 Card=82 Bytes=9676)

   2    1     HASH JOIN (Cost=5 Card=82 Bytes=8938)

   3    2       TABLE ACCESS (FULL) OF 'EMP' (Cost=2 Card=82 Bytes=7134)

   4    2       TABLE ACCESS (FULL) OF 'DEPT' (Cost=2 Card=82 Bytes=1804)

   5    1     TABLE ACCESS (FULL) OF 'DEPT1' (Cost=2 Card=82 Bytes=738)

 

Statistics

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

          0  recursive calls

          0  db block gets

         10  consistent gets

          0  physical reads

          0  redo size

       1195  bytes sent via SQL*Net to client

        503  bytes received via SQL*Net from client

          2  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

         11  rows processed

 

SQL> exec dbms_stats.gather_schema_stats('scott')

 

PL/SQL procedure successfully completed.

 

SQL> select a.*

  2  from emp a,dept b where a.deptno = b.deptno and exists (select 1 from dept1 where dept1.dname = b.dname);

11 rows selected.

 

Execution Plan

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

   0      SELECT STATEMENT ptimizer=CHOOSE (Cost=8 Card=11 Bytes=682)

   1    0   HASH JOIN (SEMI) (Cost=8 Card=11 Bytes=682)

   2    1     HASH JOIN (Cost=5 Card=11 Bytes=572)

   3    2       TABLE ACCESS (FULL) OF 'DEPT' (Cost=2 Card=4 Bytes=52)

   4    2       TABLE ACCESS (FULL) OF 'EMP' (Cost=2 Card=11 Bytes=429)

   5    1     TABLE ACCESS (FULL) OF 'DEPT1' (Cost=2 Card=4 Bytes=40)

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