Oracle绑定变量Bind Peeking
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_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 (%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_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID 3rg5r8sghcvb3, child number 0
-------------------------------------
select count(*) from acs_test_tab where record_type = :v
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。