直方圖；繫結變數窺探；自適應遊標

阿新 • • 發佈：2019-02-11

搭建環境

構建一個表
CREATE TABLE TEST2 AS SELECT * FROM DBA_OBJECTS;
UPDATE SET OBJECT_ID = 11111 WHERE ROWNUM <=50000;
建立索引：
CREATE INDEX IND_TEST2_OBJECT_ID ON TEST2(OBJECT_ID);
收集統計資訊，不收集直方圖
EXEC DBMS_STATS.GATHER_TABLE_STATS(OWNNAME=>'SCOTT',TABNAME=>'TEST2',ESTIMATE_PERCENT=>100,cascade=>true,method_opt => 'for all columns size 1');

SQL> SELECT OBJECT_ID FROM TEST2 WHERE OBJECT_NAME = 'TEST';
 OBJECT_ID
----------
     52694

毋庸置疑index range scan
SELECT count(*) FROM TEST2 WHERE OBJECT_ID=11111;
SELECT count(*) FROM TEST2 WHERE OBJECT_ID=52694;

收集直方圖

BEGIN
  DBMS_STATS.GATHER_TABLE_STATS(ownname => 'SCOTT',
                                tabname => 'TEST2',
                                estimate_percent => 100,
                                method_opt => 'for all columns size skewonly',
                                no_invalidate => FALSE,
                                degree => 1,
                                cascade => TRUE);
END;
/

全表掃描
SELECT count(*) FROM TEST2 WHERE OBJECT_ID=11111;
index range scan
SELECT count(*) FROM TEST2 WHERE OBJECT_ID=52694;

但當直方圖碰上繫結變數：
9i，10g

alter system flush shared_pool;
var a number;
exec :a := 52694;
SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
         1
SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
         1
SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
         1
SQL> select sql_id,child_number from v$sql where sql_text like 'SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a%';
SQL_ID        CHILD_NUMBER
------------- ------------
22gd2utvtttmj            0
SQL> select * from table(dbms_xplan.display_cursor('22gd2utvtttmj',0,'advanced peeked_binds'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  22gd2utvtttmj, child number 0
-------------------------------------
SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a

Plan hash value: 3600962442

-----------------------------------------------------------------------------------------
| Id  | Operation         | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |                     |       |       |     1 (100)|          |
|   1 |  SORT AGGREGATE   |                     |     1 |     5 |            |          |
|*  2 |   INDEX RANGE SCAN| IND_TEST2_OBJECT_ID |     1 |     5 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

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

   1 - SEL$1
   2 - SEL$1 /  
[email protected]$1

Outline Data
-------------

  /*+
      BEGIN_OUTLINE_DATA
      IGNORE_OPTIM_EMBEDDED_HINTS
      OPTIMIZER_FEATURES_ENABLE('10.2.0.1')
      ALL_ROWS
      OUTLINE_LEAF(@"SEL$1")
      INDEX(@"SEL$1" "TEST2"@"SEL$1" ("TEST2"."OBJECT_ID"))
      END_OUTLINE_DATA
  */

Peeked Binds (identified by position):
--------------------------------------

   1 - :A (NUMBER): 52694

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

   2 - access("OBJECT_ID"=:A)

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

   1 - (#keys=0) COUNT(*)[22]
==================================================================================================================

exec :a := 11111;
SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
     50000
SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
     50000
SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
     50000
SQL> select sql_id,child_number from v$sql where sql_text like 'SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a%';

SQL_ID        CHILD_NUMBER
------------- ------------
22gd2utvtttmj            0

SQL>  select * from table(dbms_xplan.display_cursor('22gd2utvtttmj',0,'advanced peeked_binds'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  22gd2utvtttmj, child number 0
-------------------------------------
SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a

Plan hash value: 3600962442

-----------------------------------------------------------------------------------------
| Id  | Operation         | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |                     |       |       |     1 (100)|          |
|   1 |  SORT AGGREGATE   |                     |     1 |     5 |            |          |
|*  2 |   INDEX RANGE SCAN| IND_TEST2_OBJECT_ID |     1 |     5 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

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

   1 - SEL$1
   2 - SEL$1 / [email protected]$1

Outline Data
-------------

  /*+
      BEGIN_OUTLINE_DATA
      IGNORE_OPTIM_EMBEDDED_HINTS
      OPTIMIZER_FEATURES_ENABLE('10.2.0.1')
      ALL_ROWS
      OUTLINE_LEAF(@"SEL$1")
      INDEX(@"SEL$1" "TEST2"@"SEL$1" ("TEST2"."OBJECT_ID"))
      END_OUTLINE_DATA
  */

Peeked Binds (identified by position):
--------------------------------------

   1 - :A (NUMBER): 52694

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

   2 - access("OBJECT_ID"=:A)

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

   1 - (#keys=0) COUNT(*)[22]

從上面可以發現，由於繫結變數只有在硬解析的時候才回去“窺探”，所以硬解析時候窺探的繫結變數的值就決定了整個執行計劃，即使發生改變oracle的也不管。這個就比較坑爹了。
11g

SQL> var a number;
SQL> exec :a := 52694;
PL/SQL procedure successfully completed.
SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
         1
SQL>  SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
         1
SQL>  SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
         1

SQL> select * from table(dbms_xplan.display_cursor(null,null,'advanced peeked_binds'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  67wpp1xx1jsb4, child number 0
-------------------------------------
 SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a

Plan hash value: 3600962442

-----------------------------------------------------------------------------------------
| Id  | Operation         | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |                     |       |       |     1 (100)|          |
|   1 |  SORT AGGREGATE   |                     |     1 |     5 |            |          |
|*  2 |   INDEX RANGE SCAN| IND_TEST2_OBJECT_ID |     1 |     5 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

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

   1 - SEL$1
   2 - SEL$1 / [email protected]$1

Outline Data
-------------

  /*+
      BEGIN_OUTLINE_DATA
      IGNORE_OPTIM_EMBEDDED_HINTS
      OPTIMIZER_FEATURES_ENABLE('11.2.0.3')
      DB_VERSION('11.2.0.3')
      ALL_ROWS
      OUTLINE_LEAF(@"SEL$1")
      INDEX(@"SEL$1" "TEST2"@"SEL$1" ("TEST2"."OBJECT_ID"))
      END_OUTLINE_DATA
  */

Peeked Binds (identified by position):
--------------------------------------

   1 - :A (NUMBER): 52694

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

   2 - access("OBJECT_ID"=:A)

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

   1 - (#keys=0) COUNT(*)[22]

==============================================================================
SQL> exec :a := 11111;
PL/SQL procedure successfully completed.
SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
     50000
SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
     50000
SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;
  COUNT(*)
----------
     50000
SQL> select sql_id,child_number from v$sql where sql_text like 'SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a%';

SQL_ID        CHILD_NUMBER
------------- ------------
22gd2utvtttmj            0
22gd2utvtttmj            1
22gd2utvtttmj            2

SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;

  COUNT(*)
----------
     50000

SQL>  select * from table(dbms_xplan.display_cursor(null,null,'advanced peeked_binds'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  22gd2utvtttmj, child number 2
-------------------------------------
SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a

Plan hash value: 1970853342

---------------------------------------------------------------------------------------------
| Id  | Operation             | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                     |       |       |    48 (100)|          |
|   1 |  SORT AGGREGATE       |                     |     1 |     5 |            |          |
|*  2 |   INDEX FAST FULL SCAN| IND_TEST2_OBJECT_ID | 49956 |   243K|    48   (3)| 00:00:01 |
---------------------------------------------------------------------------------------------

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

   1 - SEL$1
   2 - SEL$1 / [email protected]$1

Outline Data
-------------

  /*+
      BEGIN_OUTLINE_DATA
      IGNORE_OPTIM_EMBEDDED_HINTS
      OPTIMIZER_FEATURES_ENABLE('11.2.0.3')
      DB_VERSION('11.2.0.3')
      ALL_ROWS
      OUTLINE_LEAF(@"SEL$1")
      INDEX_FFS(@"SEL$1" "TEST2"@"SEL$1" ("TEST2"."OBJECT_ID"))
      END_OUTLINE_DATA
  */

Peeked Binds (identified by position):
--------------------------------------

   1 - :A (NUMBER): 11111

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

   2 - filter("OBJECT_ID"=:A)

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

   1 - (#keys=0) COUNT(*)[22]


49 rows selected.

SQL>  exec :a := 52694;

PL/SQL procedure successfully completed.

SQL> SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a;

  COUNT(*)
----------
         1

SQL>  select * from table(dbms_xplan.display_cursor(null,null,'advanced peeked_binds'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  22gd2utvtttmj, child number 1
-------------------------------------
SELECT count(*) FROM TEST2 WHERE OBJECT_ID=:a

Plan hash value: 3600962442

-----------------------------------------------------------------------------------------
| Id  | Operation         | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |                     |       |       |     1 (100)|          |
|   1 |  SORT AGGREGATE   |                     |     1 |     5 |            |          |
|*  2 |   INDEX RANGE SCAN| IND_TEST2_OBJECT_ID |     1 |     5 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

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

   1 - SEL$1
   2 - SEL$1 / [email protected]$1

Outline Data
-------------

  /*+
      BEGIN_OUTLINE_DATA
      IGNORE_OPTIM_EMBEDDED_HINTS
      OPTIMIZER_FEATURES_ENABLE('11.2.0.3')
      DB_VERSION('11.2.0.3')
      ALL_ROWS
      OUTLINE_LEAF(@"SEL$1")
      INDEX(@"SEL$1" "TEST2"@"SEL$1" ("TEST2"."OBJECT_ID"))
      END_OUTLINE_DATA
  */

Peeked Binds (identified by position):
--------------------------------------

   1 - :A (NUMBER): 75819

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

   2 - access("OBJECT_ID"=:A)

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

   1 - (#keys=0) COUNT(*)[22]

從上面可以看出來，11g之後，oracle引入自適應遊標，oracle會根據繫結變數值得改變而改變執行計劃了。這才是我們需要的。

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直方圖；繫結變數窺探；自適應遊標

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