Oracle中使用DBMS_XPLAN处理执行计划详解

DBMS_XPLAN是Oracle提供的一个用于查看sql计划，包括执行计划和解释计划的包；在以前查看sql执行计划的时候，我都是使用set autotrace命令，不过现在看来，DBMS_XPLAN包给出了更加简化的获取和显示计划的方式。

这5个函数分别对应不同的显示计划的方式，DBMS_XPLAN包不仅可以获取解释计划，它还可以用来输出存储在AWR，sql调试集，缓存的sql游标，以及sql基线中的语句计划，实现如上的功能，通常会用到一下5个方法：

1.disPLAY
2.disPLAY_AWR
3.disPLAY_CURSOR
4.disPLAY_PLAN
5.disPLAY_sql_PLAN_BASELINE
6.disPLAY_sqlSET

下面将重点讨论关于DBMS_XPLAN包在解释计划和执行计划上的应用。

来看一个经常使用的查看某条语句的解释计划示例：

代码如下:

sql> explain plan for select * from scott.emp;

Explained.

sql> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 532 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| EMP | 14 | 532 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------

8 rows selected.

-- 对应autotrace实现
sql> set autotrace traceonly explain

上例中使用了dbms_xplan.display方法来显示PLAN_TABLE中保存的解释计划，如果想要显示执行计划，就需要使用到DMBS_XPLAN.disPLAY_CURSOR方法了，DMBS_XPLAN.disPLAY_CURSOR调用签名：

代码如下:

DBMS_XPLAN.disPLAY_CURSOR(
sql_id IN VARCHAR2 DEFAULT NULL,
child_number IN NUMBER DEFAULT NULL,
format IN VARCHAR2 DEFAULT 'TYPICAL');

sql_id表示存储在cursor cache中的sql语句的id，child_number用于指示缓存sql语句计划的子id，format参数用于控制包含在输出中的信息类型，官档的参数如下：

1.BASIC: 显示最少的信息，只包括操作类型，ID名称和选项。
2.TYPICAL: 默认值，显示相关信息以及某些附加的显示选项，如分区和并发使用等。
3.SERIAL: 与TYPICAL类型相似，区别是它不包括并发的信息，即使是并行执行的计划。
4.ALL: 显示最多的信息，包含了TYPICAL的全部以及更多的附加信息,如别名和远程调用等。

除了以上的基本的四种输出格式外，format还有一些附加的选项可用于定制化输出行为，使用中可以通过逗号和空格分隔来声明多个关键字，同时可以使用”+”和”-”符号来包含或排除相应的显示元素，这些附加的选项在官档中也有记载：

1.ROWS – 显示被优化器估算的记录的行号
2.BYTES – 显示优化器估算的字节数
3.COST – 显示优化器计算的成本信息
4.PARTITION – 显示分区的分割信息
5.ParaLLEL – 显示并行执行信息
6.PREDICATE – 显示谓语
7.PROJECTION – 显示列投影部分(每一行的那些列被传递给其父列已经这些列的大小)
8.ALIAS – 显示查询块名称已经对象别名
9.REMOTE – 显示分布式查询信息
10.NOTE – 显示注释
11.IOSTATS – 显示游标执行的IO统计信息
12.MEMSTATS – 为内存密集运算如散列联结，排序，或一些类型的位图运算显示内存管理统计信息
13.ALLSTATS – 与'IOSTATS MEMSTATS'等价
14.LAST – 显示最后执行的执行计划统计信息，默认显示为ALL类型，并且可以累积。

以上的参数同样适用于解释计划的display方法。

示例部分：

一、使用display_cursor方法查看最近一条语句的执行计划

代码如下:

sql> select /*+ gather_plan_statistics */ count(*) from scott.emp;

COUNT(*)
----------
14

sql> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------
sql_ID f9qyz8s3c2c02,child number 0
-------------------------------------
select /*+ gather_plan_statistics */ count(*) from scott.emp

Plan hash value: 2937609675

-------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 1 |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 1 |
| 2 | INDEX FULL SCAN| PK_EMP | 1 | 14 | 14 |00:00:00.01 | 1 |
-------------------------------------------------------------------------------------

14 rows selected.

使用dbms_xplan.display_cursor(null,'ALLSTATS LAST')时，将sql_id和child_number设置成null，表示获取上一条执行语句的执行计划；注意上面的例子中一定要指定gather_plan_statistics提示或者手动设置数据库STATISTICS_LEVEL参数为ALL来使得其抓取行数据源的执行统计信息，这些信息包括行数，一直读取次数，物理读次数，物理写次数以及运算在一行数据上耗费的运行时间，如果没有指定该提示，就不会有A-Rows,A-Time,Buffers这三列信息。

二、获取某条指定语句的执行计划

代码如下:

sql> select /*+ gather_plan_statistics */ count(*) from hr.employees where department_id in (90,100);

COUNT(*)
----------
9

-- 通过v$sql视图查询到sql语句的sql_ID和CHILD_NUMBER
sql> select sql_id,child_number,sql_text from v$sql
2 where sql_text like '%select /*+ gather_plan_statistics */ count(*)%';

sql_ID CHILD_NUMBER sql_TEXT
------------- ------------ ----------------------------------------------------------------------------------------------------
5qxmkvh40yw0p 0 select /*+ gather_plan_statistics */ count(*) from hr.employees where department_id in (90,100)
bqjrnskvpv51n 0 select sql_id,sql_text from v$sql where sql_text like '%select /*+ gather_plan_statisti
cs */ count(*)%'

-- 获取对应的执行计划
sql> select * from table(dbms_xplan.display_cursor('5qxmkvh40yw0p','ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
sql_ID 5qxmkvh40yw0p,child number 0
-------------------------------------
select /*+ gather_plan_statistics */ count(*) from hr.employees where
department_id in (90,100)

Plan hash value: 4167091351

--------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 2 |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 2 |
| 2 | INLIST IteraTOR | | 1 | | 9 |00:00:00.01 | 2 |
|* 3 | INDEX RANGE SCAN| EMP_DEPARTMENT_IX | 2 | 9 | 9 |00:00:00.01 | 2 |
--------------------------------------------------------------------------------------------------

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

3 - access(("DEPARTMENT_ID"=90 OR "DEPARTMENT_ID"=100))

21 rows selected.

三、通过format参数定制执行计划输出信息

代码如下:

-- 使用ALL来显示解释计划的全部信息
sql> explain plan for
2 select * from emp e,dept d
3 where e.deptno = d.deptno
4 and e.ename = 'JOnes' ;

Explained.

sql> select * from table(dbms_xplan.display(format=>'ALL'));

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
Plan hash value: 3625962092

----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 58 | 4 (0)| 00:00:01 |
| 1 | nesTED LOOPS | | | | | |
| 2 | nesTED LOOPS | | 1 | 58 | 4 (0)| 00:00:01 |
|* 3 | TABLE ACCESS FULL | EMP | 1 | 38 | 3 (0)| 00:00:01 |
|* 4 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| 00:00:01 |
| 5 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 20 | 1 (0)| 00:00:01 |

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

1 - SEL$1
3 - SEL$1 / E@SEL$1
4 - SEL$1 / D@SEL$1
5 - SEL$1 / D@SEL$1

Predicate information (identified by operation id):

3 - filter("E"."ENAME"='JOnes')
4 - access("E"."DEPTNO"="D"."DEPTNO")

Column Projection information (identified by operation id):
-----------------------------------------------------------

1 - (#keys=0) "E"."EMPNO"[NUMBER,22],"E"."ENAME"[VARCHAR2,10],
"E"."JOB"[VARCHAR2,9],"E"."MGR"[NUMBER,"E"."HIREDATE"[DATE,7],
"E"."SAL"[NUMBER,"E"."COMM"[NUMBER,"E"."DEPTNO"[NUMBER,
"D"."DEPTNO"[NUMBER,"D"."DNAME"[VARCHAR2,14],"D"."LOC"[VARCHAR2,13]
2 - (#keys=0) "E"."EMPNO"[NUMBER,
"D".ROWID[ROWID,"D"."DEPTNO"[NUMBER,22]
3 - "E"."EMPNO"[NUMBER,"E"."JOB"[VARCHAR2,
"E"."MGR"[NUMBER,"E"."SAL"[NUMBER,
"E"."COMM"[NUMBER,22]
4 - "D".ROWID[ROWID,22]
5 - "D"."DNAME"[VARCHAR2,13]

43 rows selected.

-- 去除执行计划上的字节数和成本统计信息
sql> select empno,ename from emp e,dept d
2 where e.deptno = d.deptno
3 and e.ename = 'JOnes' ;

EMPNO ENAME
---------- ----------
7566 JOnes

sql> select * from table(dbms_xplan.display_cursor(null,format=>'ALLSTATS LAST -COST -BYTES'));

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
sql_ID 3mypf7d6npa97,child number 1
-------------------------------------
select empno,dept d where e.deptno = d.deptno and
e.ename = 'JOnes'

Plan hash value: 3956160932

------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 8 |
|* 1 | TABLE ACCESS FULL| EMP | 1 | 1 | 1 |00:00:00.01 | 8 |
------------------------------------------------------------------------------------

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

1 - filter(("E"."ENAME"='JOnes' AND "E"."DEPTNO" IS NOT NULL))

19 rows selected.

-- 另一种选项，窥视绑定变量的值，非常方便！！
sql> variable v_empno number
sql> exec :v_empno := 7566 ;

PL/sql procedure successfully completed.

sql> select * from emp where empno = :v_empno ;

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7566 JOnes MANAGER 7839 02-APR-81 2975 20

sql> select * from table(dbms_xplan.display_cursor(null,format=>'+PEEKED_BINDS'));

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
sql_ID 9q17w9umt58m7,child number 0
-------------------------------------
select * from emp where empno = :v_empno

Plan hash value: 2949544139

--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 1 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 38 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 0 (0)| |
--------------------------------------------------------------------------------------

Peeked Binds (identified by position):
--------------------------------------

1 - :V_EMPNO (NUMBER): 7566

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

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
2 - access("EMPNO"=:V_EMPNO)

24 rows selected.

-- 并行查询信息筛选
sql> select /*+ parallel(d,4) parallel (e,4) */
2 d.dname,avg(e.sal),max(e.sal)
3 from dept d,emp e
4 where d.deptno = e.deptno
5 group by d.dname
6 order by max(e.sal),avg(e.sal) desc;

DNAME AVG(E.SAL) MAX(E.SAL)
-------------- ---------- ----------
SALES 1566.66667 2850
RESEARCH 2175 3000
ACCOUNTING 2916.66667 5000

sql> select * from table(dbms_xplan.display_cursor(null,'TYPICAL -BYTES -COST'));

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
sql_ID gahr597f78j0d,child number 0
-------------------------------------
select /*+ parallel(d,4) */ d.dname,
max(e.sal) from dept d,emp e where d.deptno = e.deptno group by
d.dname order by max(e.sal),avg(e.sal) desc

Plan hash value: 3078011448

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | | | |
| 1 | PX COORDINATOR | | | | | | |
| 2 | PX SEND QC (ORDER) | :TQ10004 | 4 | 00:00:01 | Q1,04 | P->S | QC (ORDER) |
| 3 | SORT ORDER BY | | 4 | 00:00:01 | Q1,04 | Pcwp | |
| 4 | PX RECEIVE | | 4 | 00:00:01 | Q1,04 | Pcwp | |
| 5 | PX SEND RANGE | :TQ10003 | 4 | 00:00:01 | Q1,03 | P->P | RANGE |
| 6 | HASH GROUP BY | | 4 | 00:00:01 | Q1,03 | Pcwp | |
| 7 | PX RECEIVE | | 14 | 00:00:01 | Q1,03 | Pcwp | |
| 8 | PX SEND HASH | :TQ10002 | 14 | 00:00:01 | Q1,02 | P->P | HASH |
|* 9 | HASH JOIN BUFFERED | | 14 | 00:00:01 | Q1,02 | Pcwp | |
| 10 | PX RECEIVE | | 4 | 00:00:01 | Q1,02 | Pcwp | |
| 11 | PX SEND HASH | :TQ10000 | 4 | 00:00:01 | Q1,00 | P->P | HASH |
| 12 | PX BLOCK IteraTOR | | 4 | 00:00:01 | Q1,00 | PCWC | |
|* 13 | TABLE ACCESS FULL| DEPT | 4 | 00:00:01 | Q1,00 | Pcwp | |
| 14 | PX RECEIVE | | 14 | 00:00:01 | Q1,02 | Pcwp | |
| 15 | PX SEND HASH | :TQ10001 | 14 | 00:00:01 | Q1,01 | P->P | HASH |
| 16 | PX BLOCK IteraTOR | | 14 | 00:00:01 | Q1,01 | PCWC | |
|* 17 | TABLE ACCESS FULL| EMP | 14 | 00:00:01 | Q1,01 | Pcwp | |
--------------------------------------------------------------------------------------------------

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

9 - access("D"."DEPTNO"="E"."DEPTNO")
13 - access(:Z>=:Z AND :Z<=:Z) 17 - access(:Z>=:Z AND :Z<=:Z)

38 rows selected.

DBMS_XPLAN 执行计划

Oracle中使用DBMS_XPLAN处理执行计划详解

一、使用display_cursor方法查看最近一条语句的执行计划

二、获取某条指定语句的执行计划

三、通过format参数定制执行计划输出信息

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