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Oracle绑定变量Bind Peeking

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Jun 07, 2016 pm 05:34 PM

oracle

Oracle 在9i之后引入了Bind Peeking，通过Bind Peeking，Oracle可以在硬解析的时候窥探绑定变量的值，并根据当前绑定变量的值生成

Oracle 在9i之后引入了Bind Peeking，通过Bind Peeking，Oracle可以在硬解析的时候窥探绑定变量的值，，并根据当前绑定变量的值生成执行计划。在Oracle 9i之前的版本中，Oracle仅仅通过统计信息来生成执行计划。

下面看一下不同版本Oracle下绑定变量对执行计划的影响

SQL> alter system flush shared_pool;

系统已更改。

SQL> alter system set optimizer_features_enable='8.1.7';

系统已更改。

SQL> var v number;
SQL> exec :v := 1;

PL/SQL 过程已成功完成。

SQL> select count(*) from acs_test_tab where record_type = :v;

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

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

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID 3rg5r8sghcvb3, child number 0
-------------------------------------
select count(*) from acs_test_tab where record_type = :v

Plan hash value: 2956728990

--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 3 |
| 1 | SORT AGGREGATE | | 1 | 4 | |
|* 2 | INDEX RANGE SCAN| ACS_TEST_TAB_RECORD_TYPE_I | 2 | 8 | 3 |
--------------------------------------------------------------------------------

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

2 - access("RECORD_TYPE"=:V)

已选择47行。

SQL> alter system flush shared_pool;

系统已更改。

SQL> alter system set optimizer_features_enable='11.2.0.3.1';

系统已更改。

SQL> var v number;
SQL> exec :v := 1;

PL/SQL 过程已成功完成。

SQL> select count(*) from acs_test_tab where record_type = :v;

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

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

Plan hash value: 2956728990

------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 3 (100)| |
| 1 | SORT AGGREGATE | | 1 | 4 | | |
|* 2 | INDEX RANGE SCAN| ACS_TEST_TAB_RECORD_TYPE_I | 1 | 4 | 3 (0)| 00:00:01 |
------------------------------------------------------------------------------------------------

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

1 - :V (NUMBER): 1 --绑定变量窥探

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

2 - access("RECORD_TYPE"=:V)

已选择49行。

SQL> alter system flush shared_pool;

系统已更改。

SQL> exec :v := 2;

PL/SQL 过程已成功完成。

SQL> select count(*) from acs_test_tab where record_type = :v;

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

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

Plan hash value: 2957754476

----------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 136 (100)| |
| 1 | SORT AGGREGATE | | 1 | 4 | | |
|* 2 | INDEX FAST FULL SCAN| ACS_TEST_TAB_RECORD_TYPE_I | 48031 | 187K| 136 (1)| 00:00:02 |
----------------------------------------------------------------------------------------------------

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

1 - :V (NUMBER): 2 --绑定变量窥探，绑定变量会影响最初硬解析的执行计划

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

2 - filter("RECORD_TYPE"=:V)

已选择49行。

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