禁止CBO进行子查询扩展

用hint NO_UNNEST 可以禁止CBO 进行 Subquery Unnesting 

hint UNNEST 可以提示CBO进行Subquery Unnesting


1.NO_UNNEST

SQL> set linesize 200
SQL> set pagesize 200
SQL> explain plan for select count(*) from test o where object_id>(select avg(object_id) from test i where i.object_type=o.object_type);

已解释。

SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 731730653

---------------------------------------------------------------------------------
| Id  | Operation	      | Name	| Rows	| Bytes | Cost (%CPU)| Time	|
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      | 	|     1 |    38 |   699   (2)| 00:00:09 |
|   1 |  SORT AGGREGATE       | 	|     1 |    38 |	     |		|
|*  2 |   HASH JOIN	      | 	| 53949 |  2002K|   699   (2)| 00:00:09 |
|   3 |    VIEW 	      | VW_SQ_1 |    45 |  1080 |   350   (2)| 00:00:05 |
|   4 |     HASH GROUP BY     | 	|    45 |   630 |   350   (2)| 00:00:05 |
|   5 |      TABLE ACCESS FULL| TEST	| 86974 |  1189K|   347   (1)| 00:00:05 |
|   6 |    TABLE ACCESS FULL  | TEST	| 86974 |  1189K|   347   (1)| 00:00:05 |
---------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ITEM_1"="O"."OBJECT_TYPE")
       filter("OBJECT_ID">"AVG(OBJECT_ID)")

已选择19行。




真实执行计划:

SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID	anxr90rp27yn9, child number 0
-------------------------------------
select count(*) from test o where object_id>(select avg(object_id) from
test i where i.object_type=o.object_type)

Plan hash value: 731730653

----------------------------------------------------------------------------------------------------------------------
| Id  | Operation	      | Name	| Starts | E-Rows | A-Rows |   A-Time	| Buffers |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      | 	|      1 |	  |	 1 |00:00:01.13 |   39796 |	  |	  |	     |
|   1 |  SORT AGGREGATE       | 	|      1 |	1 |	 1 |00:00:01.13 |   39796 |	  |	  |	     |
|*  2 |   HASH JOIN	      | 	|      1 |  53949 |    692K|00:00:01.09 |   39796 |  1229K|  1229K| 1557K (0)|
|   3 |    VIEW 	      | VW_SQ_1 |      1 |     45 |	45 |00:00:00.49 |   19898 |	  |	  |	     |
|   4 |     HASH GROUP BY     | 	|      1 |     45 |	45 |00:00:00.49 |   19898 |   123M|    10M| 2522K (0)|
|   5 |      TABLE ACCESS FULL| TEST	|      1 |  86974 |   1391K|00:00:00.16 |   19898 |	  |	  |	     |
|   6 |    TABLE ACCESS FULL  | TEST	|      1 |  86974 |   1391K|00:00:00.16 |   19898 |	  |	  |	     |
----------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ITEM_1"="O"."OBJECT_TYPE")
       filter("OBJECT_ID">"AVG(OBJECT_ID)")


已选择25行。

已用时间:  00: 00: 00.03




执行耗时:

SQL> select count(*) from test o where object_id>(select avg(object_id) from test i where i.object_type=o.object_type);

  COUNT(*)
----------
    692912

已用时间:  00: 00: 00.54



禁止cbo进行查询转换:


select count(*) from test o where object_id>(select  /*+ NO_UNNEST */ avg(object_id) from test i where i.object_type=o.object_type);


SQL> explain plan for 
select count(*) from test o where object_id>(select  /*+ NO_UNNEST */ avg(object_id) from test i where i.object_type=o.object_type);
  2  
已解释。

SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 2139949240

-----------------------------------------------------------------------------
| Id  | Operation	     | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |	    |	  1 |	 14 | 15960   (1)| 00:03:12 |
|   1 |  SORT AGGREGATE      |	    |	  1 |	 14 |		 |	    |
|*  2 |   FILTER	     |	    |	    |	    |		 |	    |
|   3 |    TABLE ACCESS FULL | TEST | 86974 |  1189K|	347   (1)| 00:00:05 |
|   4 |    SORT AGGREGATE    |	    |	  1 |	 14 |		 |	    |
|*  5 |     TABLE ACCESS FULL| TEST |  1933 | 27062 |	347   (1)| 00:00:05 |
-----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("OBJECT_ID"> (SELECT /*+ NO_UNNEST */ AVG("OBJECT_ID")
	      FROM "TEST" "I" WHERE "I"."OBJECT_TYPE"=:B1))
   5 - filter("I"."OBJECT_TYPE"=:B1)

已选择19行。

SQL> 
select count(*) from test o where object_id>(select  /*+ NO_UNNEST */ avg(object_id) from test i where i.object_type=o.object_type);
SQL> 

  COUNT(*)
----------
    692912

已用时间:  00: 00: 18.45
SQL> SQL>  SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID	5hzdf4w2c60sf, child number 0
-------------------------------------
select count(*) from test o where object_id>(select  /*+ NO_UNNEST */
avg(object_id) from test i where i.object_type=o.object_type)

Plan hash value: 2139949240

---------------------------------------------------------------------------------------
| Id  | Operation	     | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |	    |	   1 |	      |      1 |00:00:18.45 |	 4516K|
|   1 |  SORT AGGREGATE      |	    |	   1 |	    1 |      1 |00:00:18.45 |	 4516K|
|*  2 |   FILTER	     |	    |	   1 |	      |    692K|00:00:18.41 |	 4516K|
|   3 |    TABLE ACCESS FULL | TEST |	   1 |	86974 |   1391K|00:00:00.16 |	19898 |
|   4 |    SORT AGGREGATE    |	    |	 226 |	    1 |    226 |00:00:17.87 |	 4496K|
|*  5 |     TABLE ACCESS FULL| TEST |	 226 |	 1933 |   1458K|00:00:17.72 |	 4496K|
---------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("OBJECT_ID">)
   5 - filter("I"."OBJECT_TYPE"=:B1)


已选择24行。

已用时间:  00: 00: 00.03