Gather more plan statistics by gather_plan_statistics hint

在10g以后我们可以通过利用gather_plan_statistics提示来了解更多的SQL执行统计信息，具体使用方法如下：

SQL> set linesize 150
SQL> set pagesize 2000
SQL> set autotrace traceonly exp

SQL> select avg(SALARY),DEPARTMENT_NAME from
employees e,departments d where e.DEPARTMENT_ID=d.DEPARTMENT_ID group by DEPARTMENT_NAME;

Execution Plan
----------------------------------------------------------
Plan hash value: 3294250112

---------------------------------------------------------------------------------------------
| Id  | Operation		      | Name	    | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	      | 	    |	 27 |	621 |	  5  (20)| 00:00:01 |
|   1 |  HASH GROUP BY		      | 	    |	 27 |	621 |	  5  (20)| 00:00:01 |
|   2 |   NESTED LOOPS		      | 	    |	106 |  2438 |	  4   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL	      | EMPLOYEES   |	107 |	749 |	  3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |	  1 |	 16 |	  1   (0)| 00:00:01 |
|*  5 |     INDEX UNIQUE SCAN	      | DEPT_ID_PK  |	  1 |	    |	  0   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

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

   5 - access("E"."DEPARTMENT_ID"="D"."DEPARTMENT_ID")

SQL> show parameter cursor_sharing

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
cursor_sharing			     string	 EXACT

SQL>  show parameter statistics_level

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
statistics_level		     string	 TYPICAL

SQL> set autotrace off;

SQL> select /*+ gather_plan_statistics */   avg(SALARY),DEPARTMENT_NAME from
employees e,departments d where e.DEPARTMENT_ID=d.DEPARTMENT_ID group by DEPARTMENT_NAME;

SQL> select * from TABLE(dbms_xplan.display_cursor(NULL,NULL,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID	bctzu9xuxay18, child number 0
-------------------------------------
select /*+ gather_plan_statistics */  avg(SALARY),DEPARTMENT_NAME from employees e,departments d
where e.DEPARTMENT_ID=d.DEPARTMENT_ID group by DEPARTMENT_NAME

Plan hash value: 3294250112
-------------------------------------------------------------------------------------------------------
| Id  | Operation		      | Name	    | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------------
|   1 |  HASH GROUP BY		      | 	    |	   1 |	   27 |     11 |00:00:00.01 |	  219 |
|   2 |   NESTED LOOPS		      | 	    |	   1 |	  106 |    106 |00:00:00.01 |	  219 |
|   3 |    TABLE ACCESS FULL	      | EMPLOYEES   |	   1 |	  107 |    107 |00:00:00.01 |	    7 |
|   4 |    TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |	 107 |	    1 |    106 |00:00:00.01 |	  212 |
|*  5 |     INDEX UNIQUE SCAN	      | DEPT_ID_PK  |	 107 |	    1 |    106 |00:00:00.01 |	  106 |
-------------------------------------------------------------------------------------------------------

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

   5 - access("E"."DEPARTMENT_ID"="D"."DEPARTMENT_ID")

/* 可以从starts列看到某种操作执行了多少次，例如这里的INDEX UNIQUE SCAN为107次 */

/*也可以通过SQL_ID来定位计划信息 */

SQL> select t.*
from v$sql s
   , table(dbms_xplan.display_cursor(s.sql_id,s.child_number,'ALL IOSTATS LAST')) t where s.sql_id = '&SQL_ID' ;

Enter value for sql_id: bctzu9xuxay18 

old   3:    , table(dbms_xplan.display_cursor(s.sql_id,s.child_number,'ALL IOSTATS LAST')) t where s.sql_id = '&SQL_ID'
new   3:    , table(dbms_xplan.display_cursor(s.sql_id,s.child_number,'ALL IOSTATS LAST')) t where s.sql_id = 'bctzu9xuxay18'

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID	bctzu9xuxay18, child number 0
-------------------------------------
select /*+ gather_plan_statistics */ avg(SALARY),DEPARTMENT_NAME from
employees e,departments d where
e.DEPARTMENT_ID=d.DEPARTMENT_ID group by DEPARTMENT_NAME

Plan hash value: 3294250112

---------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation		      | Name	    | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers |
---------------------------------------------------------------------------------------------------------------------------------------
|   1 |  HASH GROUP BY		      | 	    |	   1 |	   27 |   621 |     5  (20)| 00:00:01 |     11 |00:00:00.01 |	  219 |
|   2 |   NESTED LOOPS		      | 	    |	   1 |	  106 |  2438 |     4	(0)| 00:00:01 |    106 |00:00:00.01 |	  219 |
|   3 |    TABLE ACCESS FULL	      | EMPLOYEES   |	   1 |	  107 |   749 |     3	(0)| 00:00:01 |    107 |00:00:00.01 |	    7 |
|   4 |    TABLE ACCESS BY INDEX ROWID| DEPARTMENTS |	 107 |	    1 |    16 |     1	(0)| 00:00:01 |    106 |00:00:00.01 |	  212 |
|*  5 |     INDEX UNIQUE SCAN	      | DEPT_ID_PK  |	 107 |	    1 |       |     0	(0)|	      |    106 |00:00:00.01 |	  106 |
---------------------------------------------------------------------------------------------------------------------------------------

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

   1 - SEL$1
   3 - SEL$1 / E@SEL$1
   4 - SEL$1 / D@SEL$1
   5 - SEL$1 / D@SEL$1

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

   5 - access("E"."DEPARTMENT_ID"="D"."DEPARTMENT_ID")

Column Projection Information (identified by operation id):
-----------------------------------------------------------

   1 - "DEPARTMENT_NAME"[VARCHAR2,30], AVG("SALARY")[22]
   2 - "SALARY"[NUMBER,22], "DEPARTMENT_NAME"[VARCHAR2,30]
   3 - "SALARY"[NUMBER,22], "E"."DEPARTMENT_ID"[NUMBER,22]
   4 - "DEPARTMENT_NAME"[VARCHAR2,30]
   5 - "D".ROWID[ROWID,10]

SQL> alter session set statistics_level=ALL;
Session altered.

/* 在session级别设置statistics_level为ALL,可以为我们提供更为详尽的执行统计信息 */

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