cursor_sharing為force時帶來的一個執行計劃的假象
有時候可能存在評估的執行計劃和真實的執行計劃不一樣，檢視執行計劃的方法如下:
a.explain plan ,可透過dbms_xplan.display來檢視執行計劃
b.set autot
c.v$sql_plan
d.dbms_xplan.display_cursor
a,b的執行計劃都是根據explain plan得到的，因此都不是真實的(基表是sql_plan)，而3,4的執行計劃都是真實的(基表是 V$SQL_PLAN),cursor_sharing為force+直方圖的時候可能存在一個假象,就是set autot trace給你顯示的執行計劃很完美，而真實的執行計劃是當作繫結變數處理的,第一次查詢的時候可能存在bind peeking的影響,而之後的如果沒有新的cursor產生,那麼執行計劃都是重用第一次的,[@more@]
做了個測試如下:
SQL> conn l5m/l5m @dev107_c203
Connected.
SQL> show parameter cursor
NAME TYPE
------------------------------------ ---------------------------------
VALUE
------------------------------
cursor_sharing string
EXACT
cursor_space_for_time boolean
FALSE
open_cursors integer
500
session_cached_cursors integer
20
SQL> alter session set cursor_sharing=force;
Session altered.
SQL> create table kk1(id int);
Table created.
SQL> insert into kk1 select 1 from dual;
1 row created.
SQL> insert into kk1 select 2 from dba_objects;
63413 rows created.
SQL> create index i_kk1_1 on kk1(id);
Index created.
SQL> exec dbms_stats.gather_table_stats(user,'KK1',method_opt=>'for all columns size 254');
PL/SQL procedure successfully completed.
SQL> set autot trace
SQL> select count(*) from kk1 where id=1;
Execution Plan
----------------------------------------------------------
Plan hash value: 1026585023
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 1 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | INDEX RANGE SCAN| I_KK1_1 | 1 | 3 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=1)
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
335 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> /
Execution Plan
----------------------------------------------------------
Plan hash value: 1026585023
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 1 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | INDEX RANGE SCAN| I_KK1_1 | 1 | 3 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=1)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
335 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select count(*) from kk1 where id=2;
//這裡顯示的執行計劃是假的，真正的執行計劃是index range scan
Execution Plan
----------------------------------------------------------
Plan hash value: 1408337260
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 26 (8)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | TABLE ACCESS FULL| KK1 | 62313 | 182K| 26 (8)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("ID"=2)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
125 consistent gets
0 physical reads
0 redo size
337 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
下面的真正的執行計劃
SQL> set autot off
SQL> select sql_text,sql_id,child_number from v$sql
2 where sql_text like 'select count(*) from kk1%';
SQL_TEXT
--------------------------------------------------------------------------------
SQL_ID CHILD_NUMBER
--------------------------------------- ------------
select count(*) from kk1 where id=:"SYS_B_0"
1fnfa7pqtz9h6 0
SQL> select * from table(dbms_xplan.display_cursor('1fnfa7pqtz9h6','0'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
SQL_ID 1fnfa7pqtz9h6, child number 0
-------------------------------------
select count(*) from kk1 where id=:"SYS_B_0"
Plan hash value: 1026585023
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
|* 2 | INDEX RANGE SCAN| I_KK1_1 | 1 | 3 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=:SYS_B_0)
19 rows selected.
SQL> set autot trace
可以發現下面的執行計劃是假的，和加index(t)的提示的consistent gets是一致的
SQL> select count(*) from kk1 where id=2; //走索引邏輯讀125
Execution Plan
----------------------------------------------------------
Plan hash value: 1408337260
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 26 (8)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | TABLE ACCESS FULL| KK1 | 62313 | 182K| 26 (8)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("ID"=2)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
125 consistent gets
0 physical reads
0 redo size
337 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select /*+ index(t) */ count(*) from kk1 t where id=2;
Execution Plan
----------------------------------------------------------
Plan hash value: 1026585023
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 126 (2)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | INDEX RANGE SCAN| I_KK1_1 | 62313 | 182K| 126 (2)| 00:00:02 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=2)
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
125 consistent gets
0 physical reads
0 redo size
337 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select /*+ full(t) */ count(*) from kk1 t where id=2;
//全表邏輯讀107
Execution Plan
----------------------------------------------------------
Plan hash value: 1408337260
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 26 (8)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | TABLE ACCESS FULL| KK1 | 62313 | 182K| 26 (8)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("ID"=2)
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
107 consistent gets
0 physical reads
0 redo size
337 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
下面看看dbms_xplan.display與dbms_xplan.display_cursor兩個包的結果是否一致
SQL> explain plan set statement_id='NO' for
2 select count(*) from kk1 where id=2;
SQL> select plan_table_output
2 from table(dbms_xplan.display('PLAN_TABLE','NO','ALL'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1408337260
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 26 (8)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | TABLE ACCESS FULL| KK1 | 62313 | 182K| 26 (8)| 00:00:01 |
---------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
-------------------------------------------------------------
1 - SEL$1
2 - SEL$1 / KK1@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("ID"=2)
Column Projection Information (identified by operation id):
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
-----------------------------------------------------------
1 - (#keys=0) COUNT(*)[22]
25 rows selected.
SQL> select sql_text,sql_id,child_number from v$sql where sql_text like 'select count(*) from kk1%';
SQL_TEXT
--------------------------------------------------------------------------------
SQL_ID CHILD_NUMBER
--------------------------------------- ------------
select count(*) from kk1 where id=:"SYS_B_0"
1fnfa7pqtz9h6 0
SQL> select * from table(dbms_xplan.display_cursor('1fnfa7pqtz9h6','0'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
SQL_ID 1fnfa7pqtz9h6, child number 0
-------------------------------------
select count(*) from kk1 where id=:"SYS_B_0"
Plan hash value: 1026585023
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
|* 2 | INDEX RANGE SCAN| I_KK1_1 | 1 | 3 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=:SYS_B_0)
19 rows selected.
由於dbms_xplan.display表的基表是plan_table,而dbms_xplan.display_cursor的基表是v$sql_plan，看出結果是不一致的。
a.explain plan ,可透過dbms_xplan.display來檢視執行計劃
b.set autot
c.v$sql_plan
d.dbms_xplan.display_cursor
a,b的執行計劃都是根據explain plan得到的，因此都不是真實的(基表是sql_plan)，而3,4的執行計劃都是真實的(基表是 V$SQL_PLAN),cursor_sharing為force+直方圖的時候可能存在一個假象,就是set autot trace給你顯示的執行計劃很完美，而真實的執行計劃是當作繫結變數處理的,第一次查詢的時候可能存在bind peeking的影響,而之後的如果沒有新的cursor產生,那麼執行計劃都是重用第一次的,[@more@]
做了個測試如下:
SQL> conn l5m/l5m @dev107_c203
Connected.
SQL> show parameter cursor
NAME TYPE
------------------------------------ ---------------------------------
VALUE
------------------------------
cursor_sharing string
EXACT
cursor_space_for_time boolean
FALSE
open_cursors integer
500
session_cached_cursors integer
20
SQL> alter session set cursor_sharing=force;
Session altered.
SQL> create table kk1(id int);
Table created.
SQL> insert into kk1 select 1 from dual;
1 row created.
SQL> insert into kk1 select 2 from dba_objects;
63413 rows created.
SQL> create index i_kk1_1 on kk1(id);
Index created.
SQL> exec dbms_stats.gather_table_stats(user,'KK1',method_opt=>'for all columns size 254');
PL/SQL procedure successfully completed.
SQL> set autot trace
SQL> select count(*) from kk1 where id=1;
Execution Plan
----------------------------------------------------------
Plan hash value: 1026585023
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 1 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | INDEX RANGE SCAN| I_KK1_1 | 1 | 3 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=1)
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
335 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> /
Execution Plan
----------------------------------------------------------
Plan hash value: 1026585023
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 1 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | INDEX RANGE SCAN| I_KK1_1 | 1 | 3 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=1)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
335 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select count(*) from kk1 where id=2;
//這裡顯示的執行計劃是假的，真正的執行計劃是index range scan
Execution Plan
----------------------------------------------------------
Plan hash value: 1408337260
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 26 (8)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | TABLE ACCESS FULL| KK1 | 62313 | 182K| 26 (8)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("ID"=2)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
125 consistent gets
0 physical reads
0 redo size
337 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
下面的真正的執行計劃
SQL> set autot off
SQL> select sql_text,sql_id,child_number from v$sql
2 where sql_text like 'select count(*) from kk1%';
SQL_TEXT
--------------------------------------------------------------------------------
SQL_ID CHILD_NUMBER
--------------------------------------- ------------
select count(*) from kk1 where id=:"SYS_B_0"
1fnfa7pqtz9h6 0
SQL> select * from table(dbms_xplan.display_cursor('1fnfa7pqtz9h6','0'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
SQL_ID 1fnfa7pqtz9h6, child number 0
-------------------------------------
select count(*) from kk1 where id=:"SYS_B_0"
Plan hash value: 1026585023
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
|* 2 | INDEX RANGE SCAN| I_KK1_1 | 1 | 3 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=:SYS_B_0)
19 rows selected.
SQL> set autot trace
可以發現下面的執行計劃是假的，和加index(t)的提示的consistent gets是一致的
SQL> select count(*) from kk1 where id=2; //走索引邏輯讀125
Execution Plan
----------------------------------------------------------
Plan hash value: 1408337260
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 26 (8)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | TABLE ACCESS FULL| KK1 | 62313 | 182K| 26 (8)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("ID"=2)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
125 consistent gets
0 physical reads
0 redo size
337 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select /*+ index(t) */ count(*) from kk1 t where id=2;
Execution Plan
----------------------------------------------------------
Plan hash value: 1026585023
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 126 (2)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | INDEX RANGE SCAN| I_KK1_1 | 62313 | 182K| 126 (2)| 00:00:02 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=2)
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
125 consistent gets
0 physical reads
0 redo size
337 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select /*+ full(t) */ count(*) from kk1 t where id=2;
//全表邏輯讀107
Execution Plan
----------------------------------------------------------
Plan hash value: 1408337260
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 26 (8)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | TABLE ACCESS FULL| KK1 | 62313 | 182K| 26 (8)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("ID"=2)
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
107 consistent gets
0 physical reads
0 redo size
337 bytes sent via SQL*Net to client
377 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
下面看看dbms_xplan.display與dbms_xplan.display_cursor兩個包的結果是否一致
SQL> explain plan set statement_id='NO' for
2 select count(*) from kk1 where id=2;
SQL> select plan_table_output
2 from table(dbms_xplan.display('PLAN_TABLE','NO','ALL'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1408337260
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 3 | 26 (8)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
|* 2 | TABLE ACCESS FULL| KK1 | 62313 | 182K| 26 (8)| 00:00:01 |
---------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
-------------------------------------------------------------
1 - SEL$1
2 - SEL$1 / KK1@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("ID"=2)
Column Projection Information (identified by operation id):
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
-----------------------------------------------------------
1 - (#keys=0) COUNT(*)[22]
25 rows selected.
SQL> select sql_text,sql_id,child_number from v$sql where sql_text like 'select count(*) from kk1%';
SQL_TEXT
--------------------------------------------------------------------------------
SQL_ID CHILD_NUMBER
--------------------------------------- ------------
select count(*) from kk1 where id=:"SYS_B_0"
1fnfa7pqtz9h6 0
SQL> select * from table(dbms_xplan.display_cursor('1fnfa7pqtz9h6','0'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
SQL_ID 1fnfa7pqtz9h6, child number 0
-------------------------------------
select count(*) from kk1 where id=:"SYS_B_0"
Plan hash value: 1026585023
-----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | SORT AGGREGATE | | 1 | 3 | | |
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
|* 2 | INDEX RANGE SCAN| I_KK1_1 | 1 | 3 | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=:SYS_B_0)
19 rows selected.
由於dbms_xplan.display表的基表是plan_table,而dbms_xplan.display_cursor的基表是v$sql_plan，看出結果是不一致的。
來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/271283/viewspace-1020411/，如需轉載，請註明出處，否則將追究法律責任。
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