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 ----------------------------------------------------------------------------------------------------