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：
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：
DBMS_XPLAN.DISPLAY_CURSOR(
sql_id INVARCHAR2DEFAULTNULL,
child_numberINNUMBER DEFAULTNULL,
format INVARCHAR2DEFAULT'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
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_IDf9qyz8s3c2c02, 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
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
2where 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_ID5qxmkvh40yw0p, 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
-- 使用ALL来显示解释计划的全部信息
SQL> explain plan for
2select * from emp e, dept d
3where e.deptno = d.deptno
4and 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", "E"."ENAME",
"E"."JOB", "E"."MGR", "E"."HIREDATE",
"E"."SAL", "E"."COMM", "E"."DEPTNO",
"D"."DEPTNO", "D"."DNAME", "D"."LOC"
2 - (#keys=0) "E"."EMPNO", "E"."ENAME",
"E"."JOB", "E"."MGR", "E"."HIREDATE",
"E"."SAL", "E"."COMM", "E"."DEPTNO",
"D".ROWID, "D"."DEPTNO"
3 - "E"."EMPNO", "E"."ENAME", "E"."JOB",
"E"."MGR", "E"."HIREDATE", "E"."SAL",
"E"."COMM", "E"."DEPTNO"
4 - "D".ROWID, "D"."DEPTNO"
5 - "D"."DNAME", "D"."LOC"
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
2where e.deptno = d.deptno
3and 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_ID3mypf7d6npa97, 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_ID9q17w9umt58m7, 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) */
2d.dname, avg(e.sal), max(e.sal)
3from dept d, emp e
4where d.deptno = e.deptno
5group by d.dname
6order 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_IDgahr597f78j0d, 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
--------------------------------------------------------------------------------------------------
| Id| Operation | Name | Rows| Time | TQ|IN-OUT| PQ Distrib |
--------------------------------------------------------------------------------------------------
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.
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