ORACLE绑定变量BINDPEEKING - 数据库编程

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。使用绑定变量窥测的好处是：可以帮助优化器在第一次硬解析时选择最优的执行计划。但是同时这也是其弊端：在第一次硬解析后，后面发生的所有解析都会使用第一次硬解析生成的执行计划，如果数据的分布是均匀的，问题不大，如果数据分布式倾斜的，那么第一次硬解析生成的执行计划未必是最优的，甚至可能是非常糟糕的。例如：

SQL> show parameter optimizer_feat

NAME				     TYPE	 VALUE
------------------------------------ ----------- ------------------------------
optimizer_features_enable	     string	 11.2.0.3.1
SQL> alter system flush shared_pool;

系统已更改。

SQL> var v number;
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);

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 |
----------------------------------------------------------------------------------------------------

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

   2 - filter("RECORD_TYPE"=:V)

已选择19行。

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);

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

ORACLE绑定变量BINDPEEKING(二)