dbms_xplan之display_cursor函数的使用

DBMS_XPLAN包中display_cursor函数不同于display函数，display_cursor用于显示SQL语句的真实的执行计划，在大多数情况下，显示真实
的执行计划有助于更好的分析SQL语句的全过程，尤其是运行此SQL语句实时的I/O开销。通过对比预估的I/O与真实的I/O开销来判断SQL语句所存
在问题，如缺少统计信息，SQL语句执行的次数，根据实际中间结果集的大小来选择合适的连接方式等。本文仅仅讲述display_cursor函数的使
用。

一、display_cursor函数用法
1、display_cursor函数语法

DBMS_XPLAN.DISPLAY_CURSOR(
sql_id IN VARCHAR2 DEFAULT NULL,
child_number IN NUMBER DEFAULT NULL,
format IN VARCHAR2 DEFAULT 'TYPICAL');
2、display_cursor函数参数描述
sql_id
指定位于库缓存执行计划中SQL语句的父游标。默认值为null。当使用默认值时当前会话的最后一条SQL语句的执行计划将被返回
可以通过查询V$SQL 或V$SQLAREA的SQL_ID列来获得SQL语句的SQL_ID。
child_number
指定父游标下子游标的序号。即指定被返回执行计划的SQL语句的子游标。默认值为0。如果为null，则sql_id所指父游标下所有子游标
的执行计划都将被返回。
format
控制SQL语句执行计划的输出部分，即哪些可以显示哪些不显示。使用与display函数的format参数与修饰符在这里同样适用。
除此之外当在开启statistics_level=all时或使用gather_plan_statistics提示可以获得执行计划中实时的统计信息
有关详细的format格式描述请参考：dbms_xplan之display函数的使用 中format参数的描述

下面给出启用统计信息时format新增的修饰符
iostats 控制I/O统计的显示
last 默认，显示所有执行计算过的统计。如果指定该值，则只显示最后一次执行的统计信息
memstats 控制pga相关统计的显示
allstats 此为iostats memstats的快捷方式，即allstats包含了iostats和memstats
run_stats_last 等同于iostats last。只能用于oracle 10g R1
run_stats_tot 等同于iostats。只能用于oracle 10g R1

二、演示使用display_cursor函数获取执行计划
1、当前数据库版本以及加载执行计划到库缓存

SQL> select * from v$version where rownum<2;

BANNER
----------------------------------------------------------------
Oracle Database 10g Release 10.2.0.3.0 - 64bit Production

SQL> SELECT ename,dname,loc
2 FROM emp e, dept d
3 WHERE e.deptno = d.deptno
4 AND e.empno = 7788;

ENAME DNAME LOC
---------- -------------- -------------
SCOTT RESEARCH DALLAS
2、查看真实的执行计划

/*----------------不传递任何参数给display_cursor函数，显示当前会话最后一条SQL语句的执行计划-------------*/
/**************************************************/
/* Author: Robinson Cheng */
/* Blog: http://blog.csdn.net/robinson_0612 */
/* MSN: robinson_0612@hotmail.com */
/* QQ: 645746311 */
/**************************************************/
SQL> select * from table(dbms_xplan.display_cursor(null,null));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------
SQL_ID a67wqmkfb9j65, child number 0
-------------------------------------
SELECT ename,dname,loc FROM emp e, dept d WHERE e.deptno = d.deptno AND
e.empno = 7788

Plan hash value: 2385808155

----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 3 (100)| |
| 1 | NESTED LOOPS | | 1 | 63 | 3 (0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 33 | 2 (0)| 00:00:01 |
|* 3 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 1 (0)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| DEPT | 409 | 12270 | 1 (0)| 00:00:01 |
|* 5 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| |
----------------------------------------------------------------------------------------

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

3 - access("E"."EMPNO"=7788)
5 - access("E"."DEPTNO"="D"."DEPTNO")

/*------------------- 获得SQL语句的SQL_ID，可以看出此SQL_ID与上面显示的执行计划中的SQL_ID一致----------*/
SQL> select sql_id,address,plan_hash_value,hash_value,child_number from v$sql
2 where sql_text like '%SELECT ename%' and sql_text not like '%from v$sql%';

SQL_ID ADDRESS PLAN_HASH_VALUE HASH_VALUE CHILD_NUMBER
------------- ---------------- --------------- ---------- ------------
a67wqmkfb9j65 0000000091DBFBC8 2385808155 2629092549 0

/*-------------- 传递SQL_ID以及format参数，并配合修饰符控制执行计划的输出------------------------*/
SQL> select * from table(dbms_xplan.display_cursor('a67wqmkfb9j65',null,'typical -predicate -rows'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------
SQL_ID a67wqmkfb9j65, child number 0
-------------------------------------
SELECT ename,dname,loc FROM emp e, dept d WHERE e.deptno = d.deptno
AND e.empno = 7788

Plan hash value: 2385808155

--------------------------------------------------------------------------------
| Id | Operation | Name | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | 3 (100)| |
| 1 | NESTED LOOPS | | 63 | 3 (0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| EMP | 33 | 2 (0)| 00:00:01 |
| 3 | INDEX UNIQUE SCAN | PK_EMP | | 1 (0)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| DEPT | 12270 | 1 (0)| 00:00:01 |
| 5 | INDEX UNIQUE SCAN | PK_DEPT | | 0 (0)| |
--------------------------------------------------------------------------------
3、查看真实执行计划并获得统计信息
前提条件
设置参数statistics_level为all,可以基于session级别以及实例级别
或者启用gather_plan_statistics提示

/*-------------查看实例参数statistics_level的值，并在会话级别将其设定为all ---------*/
SQL> show parameter statistics_le

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
statistics_level string ALL

SQL> alter session set statistics_level=all;

Session altered.

SQL> select e.ename,e.sal,s.grade
2 from emp e
3 join salgrade s
4 on e.sal between losal and hisal
5 and e.deptno = 20;

ENAME SAL GRADE
---------- ---------- ----------
SCOTT 3000 4
FORD 3000 4
JONES 2975 4
ADAMS 1100 1
SMITH 800 1

/*------- 执行上述SQL语句后获得其真实的执行计划，使用了iostats last -predicate -note 修饰符控制显示输出-----*/
SQL> set pagesize 0
SQL> select * from table(dbms_xplan.display_cursor(null,null,'iostats last -predicate -note'));
SQL_ID 243b0tpjxj6wv, child number 0
-------------------------------------
select e.ename,e.sal,s.grade from emp e join salgrade s on e.sal between losal and
hisal and e.deptno = 20

Plan hash value: 4204027666

-------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------------
| 1 | MERGE JOIN | | 1 | 1 | 5 |00:00:00.01 | 14 |
| 2 | SORT JOIN | | 1 | 5 | 5 |00:00:00.01 | 7 |
| 3 | TABLE ACCESS FULL | EMP | 1 | 5 | 5 |00:00:00.01 | 7 |
| 4 | FILTER | | 5 | | 5 |00:00:00.01 | 7 |
| 5 | SORT JOIN | | 5 | 5 | 14 |00:00:00.01 | 7 |
| 6 | TABLE ACCESS FULL| SALGRADE | 1 | 5 | 5 |00:00:00.01 | 7 |
-------------------------------------------------------------------------------------------

/*---------------- 修改会话级别的参数statistics_level为typical并验证修改结果----------------*/
SQL> alter session set statistics_level=typical;

SQL> col name format a40
SQL> col value format a25
SQL> col display_value format a25
SQL> select name, value, display_value, isses_modifiable
2 from v$parameter
3 where isses_modifiable = 'TRUE'
4 and name like '%&input_name%';
Enter value for input_name: statistics_level
old 4: and name like '%&input_name%'
new 4: and name like '%statistics_level%'

NAME VALUE DISPLAY_VALUE ISSES
---------------------------------------- ------------------------- ------------------------- -----
statistics_level TYPICAL TYPICAL TRUE

/*-------- 使用提示gather_plan_statistics，并获得其真实执行计划，使用了allstats -rows修饰符控制显示输出---*/
SQL> set pagesize 180
SQL> SELECT /*+ gather_plan_statistics */ ename,dname,loc
2 FROM emp e, dept d
3 WHERE e.deptno = d.deptno
4 AND d.deptno=20 ORDER BY 1,2,3;

ENAME DNAME LOC
---------- -------------- -------------
ADAMS RESEARCH DALLAS
FORD RESEARCH DALLAS
JONES RESEARCH DALLAS
SCOTT RESEARCH DALLAS
SMITH RESEARCH DALLAS

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats -rows'));

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------
SQL_ID d2hh42yzqqjz7, child number 0
-------------------------------------
SELECT /*+ gather_plan_statistics */ ename,dname,loc FROM emp e, dept d WHERE e.deptno = d.deptno AND
d.deptno=20 ORDER BY 1,2,3

Plan hash value: 3339094711

---------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | A-Rows | A-Time | Buffers | OMem | 1Mem | O/1/M |
---------------------------------------------------------------------------------------------------------------------
| 1 | SORT ORDER BY | | 1 | 5 |00:00:00.01 | 9 | 2048 | 2048 | 1/0/0|
| 2 | NESTED LOOPS | | 1 | 5 |00:00:00.01 | 9 | | | |
| 3 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 1 |00:00:00.01 | 2 | | | |
|* 4 | INDEX UNIQUE SCAN | PK_DEPT | 1 | 1 |00:00:00.01 | 1 | | | |
|* 5 | TABLE ACCESS FULL | EMP | 1 | 5 |00:00:00.01 | 7 | | | |
---------------------------------------------------------------------------------------------------------------------

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

4 - access("D"."DEPTNO"=20)
5 - filter("E"."DEPTNO"=20)

Note
-----
- dynamic sampling used for this statement
三、总结
1、与display函数不同，display_cursor显示的为真实的执行计划
2、对于format参数，使用与display函数的各个值，同样适用于display_cursor函数
3、当statistics_level为all或使用gather_plan_statistics提示可以获得执行时的统计信息
4、根据真实与预估的统计信息可以初步判断SQL效率低下的原因，如统计信息的准确性、主要的开销位于那些步骤等