如何快速得到真实的执行计划

准备工作：

create table zbdba as select * from dba_objects;

create table zbdba1 as select * from dba_objects;

create index zbdba_owner on zbdba(owner);

create index zbdba1_owner on zbdba1(owner);

exec dbms_stats.gather_table_stats(user, 'ZBDBA', method_opt => 'FOR ALL COLUMNS SIZE 1');

exec dbms_stats.gather_table_stats(user, 'ZBDBA1', method_opt => 'FOR ALL COLUMNS SIZE 1');

通常我们对于执行时间很长的sql查看执行计划：

explain plan for select zbdba.object_name from zbdba,zbdba1 where zbdba.owner=zbdba1.owner

[email protected]orcl11g>select plan_table_output from table(dbms_xplan.display());
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 1287183320

----------------------------------------------------------------------------------------------
| Id  | Operation             | Name         | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |              |   207M|  7330M|       |  1198  (55)| 00:00:15 |
|*  1 |  HASH JOIN            |              |   207M|  7330M|  1272K|  1198  (55)| 00:00:15 |
|   2 |   INDEX FAST FULL SCAN| ZBDBA1_OWNER | 72068 |   422K|       |    48   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL   | ZBDBA        | 72067 |  2181K|       |   288   (1)| 00:00:04 |
----------------------------------------------------------------------------------------------

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

   1 - access("ZBDBA"."OWNER"="ZBDBA1"."OWNER")

15 rows selected.

但是这种执行计划不一定是准确的。那我们怎么能才能快速的得到准确的执行计划呢？

真实的执行计划就是已经执行的sql

那么

select zbdba.object_name from zbdba,zbdba1 where zbdba.owner=zbdba1.owner;

你不需要一直等，ctrl+c中断即可

[email protected]>select sql_text,sql_id,hash_value,child_number from v$sql where sql_text like 'select zbdba.object_name from zbdba,zbdba1 where zbdba.owner=zbdba1.owner%';
SQL_TEXT                                                     SQL_ID                     HASH_VALUE CHILD_NUMBER
------------------------------------------------------------ -------------------------- ---------- ------------
select zbdba.object_name from zbdba,zbdba1 where zbdba.owner fr4g7ypwx5krq              2043857654            0
=zbdba1.owner

[email protected]>select * from table(dbms_xplan.display_cursor('fr4g7ypwx5krq',0,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  fr4g7ypwx5krq, child number 0
-------------------------------------
select zbdba.object_name from zbdba,zbdba1 where
zbdba.owner=zbdba1.owner

Plan hash value: 1287183320

----------------------------------------------------------------------------------
| Id  | Operation             | Name         | E-Rows |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |              |        |       |       |          |
|*  1 |  HASH JOIN            |              |    207M|  3024K|  1862K| 7066K (0)|
|   2 |   INDEX FAST FULL SCAN| ZBDBA1_OWNER |  72068 |       |       |          |
|   3 |   TABLE ACCESS FULL   | ZBDBA        |  72067 |       |       |          |
----------------------------------------------------------------------------------

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

   1 - access("ZBDBA"."OWNER"="ZBDBA1"."OWNER")

Note
-----
   - Warning: basic plan statistics not available. These are only collected when:
       * hint 'gather_plan_statistics' is used for the statement or
       * parameter 'statistics_level' is set to 'ALL', at session or system level

27 rows selected.

这样就快速从内存中到了真实的执行计划