ORACLE 在9i之後引入了bind peeking,通過bind peeking,oracle可以在硬解析的時候窺探繫結變數的值，並根據當前繫結變數的值生成執行計劃。在oracle 9i之前的版本中，oracle僅僅通過統計資訊來生成執行計劃。
下面看一下不同版本oracle下繫結變數對執行計劃的影響
SQL> alter sysTEM flush shared_pool;
系統已更改。
SQL> alter system set optimizer_features_enable='8.1.7';
系統已更改。
SQL> var v number;
SQL> exec :v := 1;
PL/SQL 過程已成功完成。
SQL> select count(*) from acs_test_tab where record_type = :v;
COUNT(*)
----------
1
SQL> select * from table(dbms_lay_cursor(null,null,'advanced'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID3rg5r8sghcvb3, 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)
已選擇47行。
SQL> alter system flush shared_pool;
系統已更改。
SQL> alter system set optimizer_features_enable='';
系統已更改。
SQL> var v number;
SQL> exec :v := 1;
PL/SQL 過程已成功完成。
SQL> select count(*) from acs_test_tab where record_type = :v;
COUNT(*)
----------
1
SQL> select * from table(dbms_lay_cursor(null,null,'advanced'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID3rg5r8sghcvb3, 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 --繫結變數窺探
Predicate Information (identified by operation id)：
---------------------------------------------------
2 - access("RECORD_TYPE"=:V)
已選擇49行。
SQL> alter system flush shared_pool;
系統已更改。
SQL> exec :v := 2;
PL/SQL 過程已成功完成。
SQL> select count(*) from acs_test_tab where record_type = :v;
COUNT(*)
----------
50000
SQL> select * from table(dbms_lay_cursor(null,null,'advanced'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID3rg5r8sghcvb3, 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 |
