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：
复制代码代码如下:
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：

复制代码代码如下:
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

复制代码代码如下:
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,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

复制代码代码如下:
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,child_number,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',0,'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

复制代码代码如下:
-- 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,22], "E"."HIREDATE"[DATE,7],
       "E"."SAL"[NUMBER,22], "E"."COMM"[NUMBER,22], "E"."DEPTNO"[NUMBER,22],
       "D"."DEPTNO"[NUMBER,22], "D"."DNAME"[VARCHAR2,14], "D"."LOC"[VARCHAR2,13]
   2 - (#keys=0) "E"."EMPNO"[NUMBER,22], "E"."ENAME"[VARCHAR2,10],
       "E"."JOB"[VARCHAR2,9], "E"."MGR"[NUMBER,22], "E"."HIREDATE"[DATE,7],
       "E"."SAL"[NUMBER,22], "E"."COMM"[NUMBER,22], "E"."DEPTNO"[NUMBER,22],
       "D".ROWID[ROWID,10], "D"."DEPTNO"[NUMBER,22]
   3 - "E"."EMPNO"[NUMBER,22], "E"."ENAME"[VARCHAR2,10], "E"."JOB"[VARCHAR2,9],
       "E"."MGR"[NUMBER,22], "E"."HIREDATE"[DATE,7], "E"."SAL"[NUMBER,22],
       "E"."COMM"[NUMBER,22], "E"."DEPTNO"[NUMBER,22]
   4 - "D".ROWID[ROWID,10], "D"."DEPTNO"[NUMBER,22]
   5 - "D"."DNAME"[VARCHAR2,14], "D"."LOC"[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,null,format=>'ALLSTATS LAST -COST -BYTES'));

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID 3mypf7d6npa97, child number 1
-------------------------------------
select empno, ename from emp e, 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,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,null,'TYPICAL -BYTES -COST'));

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID gahr597f78j0d, child number 0
-------------------------------------
select /*+ parallel(d, 4) parallel (e, 4) */ d.dname, avg(e.sal),
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 |            |
--------------------------------------------------------------------------------------------------