执行计划中各字段各模块描述

朱雀 2022-03-16 19:58 186阅读 0赞

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在SQL语句的执行计划中，包含很多字段项和很多模块，其不同字段代表了不同的含义且在不同的情形下某些字段、模块显示或不显示，下  
面的描述给出了执行计划中各字段的含义以及各模块的描述。

有关执行计划中各字段模块的描述请参考： [执行计划中各字段各模块描述][Link 1]  
有关由SQL语句来获取执行计划请参考： [使用 EXPLAIN PLAN 获取SQL语句执行计划][EXPLAIN PLAN _SQL]  
有关使用autotrace来获取执行计划请参考：[启用 AUTOTRACE 功能][AUTOTRACE]  
有关display\_cursor函数的使用请参考： [dbms\_xplan之display\_cursor函数的使用][dbms_xplan_display_cursor]

一、执行计划中各字段的描述  
1、基本字段(总是可用的)  
Id 执行计划中每一个操作(行)的标识符。如果数字前面带有星号，意味着将在随后提供这行包含的谓词信息  
Operation 对应执行的操作。也叫行源操作  
Name 操作的对象名称  
  
2、查询优化器评估信息  
Rows(E-Rows) 预估操作返回的记录条数  
Bytes(E-Bytes) 预估操作返回的记录字节数  
TempSpc 预估操作使用临时表空间的大小  
Cost(%CPU) 预估操作所需的开销。在括号中列出了CPU开销的百分比。注意这些值是通过执行计划计算出来的。  
换句话说，父操作的开销包含子操作的开销  
Time 预估执行操作所需要的时间(HH:MM:SS)  
  
3、分区(仅当访问分区表时下列字段可见)  
Pstart 访问的第一个分区。如果解析时不知道是哪个分区就设为KEY,KEY(I),KEY(MC),KEY(OR),KEY(SQ)  
Pstop 访问的最后一个分区。如果解析时不知道是哪个分区就设为KEY,KEY(I),KEY(MC),KEY(OR),KEY(SQ)  
  
4、并行和分布式处理(仅当使用并行或分布式操作时下列字段可见)  
Inst 在分布式操作中，指操作使用的数据库链接的名字  
TQ 在并行操作中，用于从属线程间通信的表队列  
IN-OUT 并行或分布式操作间的关系  
PQ Distrib 在并行操作中，生产者为发送数据给消费者进行的分配  
  
5、运行时统计(当设定参数statistics\_level为all或使用gather\_plan\_statistics提示时，下列字段可见)  
Starts 指定操作执行的次数  
A-Rows 操作返回的真实记录数  
A-Time 操作执行的真实时间(HH:MM:SS.FF)  
  
6、I/O 统计(当设定参数statistics\_level为all或使用gather\_plan\_statistics提示时，下列字段可见)  
Buffers 执行期间进行的逻辑读操作数量  
Reads 执行期间进行的物理读操作数量  
Writes 执行期间进行的物理写操作数量  
  
7、内存使用统计  
OMem 最优执行所需内存的预估值  
1Mem 一次通过(one-pass)执行所需内存的预估值  
0/1/M 最优/一次通过/多次通过(multipass)模式操作执行的次数  
Used-Mem 最后一次执行时操作使用的内存量  
Used-Tmp 最后一次执行时操作使用的临时空间大小。这个字段必须扩大1024倍才能和其他衡量内存的字段一致(比如，32k意味着32MB)  
Max-Tmp 操作使用的最大临时空间大小。这个字段必须扩大1024倍才能和其他衡量内存的字段一致(比如，32k意味着32MB)

二、执行计划中各模块的描述与举例  
1、预估的执行计划中的各字段与模块

SQL> explain plan for                                                                                                    2  select * from emp e,dept d                                                                                          3  where e.deptno=d.deptno                                                                                             4  and e.ename='SMITH';                                                                                                                                                                                                                     Explained.                                                                                                                                                                                                                                    /**************************************************/                                                                   /* Author: Robinson Cheng                         */                                                                   /* Blog:   http://blog.csdn.net/robinson_0612     */                                                                   /* MSN:    robinson_0612@hotmail.com              */                                                                   /* QQ:     645746311                              */                                                                   /**************************************************/                                                                                                                                                                                                                                                                                                                 SQL> set linesize 180                                                                                                  SQL> set pagesize 0                                                                                                    SQL> select * from table(dbms_xplan.display(null,null,'advanced'));   --使用dbms_xplan.display函数获得语句的执行计划   Plan hash value: 351108634                                            --SQL语句的哈希植                                                                                                                                                       ----------------------------------------------------------------------------------------   /*执行计划部分*/            | Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |                               ----------------------------------------------------------------------------------------                               |   0 | SELECT STATEMENT             |         |     1 |   117 |     4   (0)| 00:00:01 |                               |   1 |  NESTED LOOPS                |         |     1 |   117 |     4   (0)| 00:00:01 |                               |*  2 |   TABLE ACCESS FULL          | EMP     |     1 |    87 |     3   (0)| 00:00:01 |                               |   3 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     1 |    30 |     1   (0)| 00:00:01 |                               |*  4 |    INDEX UNIQUE SCAN         | PK_DEPT |     1 |       |     0   (0)| 00:00:01 |                               ----------------------------------------------------------------------------------------                                                                                                                                                      Query Block Name / Object Alias (identified by operation id):  --这部分显示的为查询块名和对象别名                      -------------------------------------------------------------                                                                                                                                                                                    1 - SEL$1                    --SEL$为select 的缩写，位于块1,相应的还有DEL$,INS$,UPD$等                                 2 - SEL$1 / E@SEL$1          --E@SEL$1，对应到执行计划中的操作ID为2上，即在表E上的查询，E为别名，下面类同              3 - SEL$1 / D@SEL$1                                                                                                    4 - SEL$1 / D@SEL$1                                                                                                                                                                                                                        Outline Data                    --提纲部分，这部分将执行计划中的图形化方式以文本形式来呈现，即转换为提示符方式         -------------                                                                                                                                                                                                                                   /*+                                                                                                                        BEGIN_OUTLINE_DATA                                                                                                     USE_NL(@"SEL$1" "D"@"SEL$1")                           --使用USE_NL提示，即嵌套循环                                    LEADING(@"SEL$1" "E"@"SEL$1" "D"@"SEL$1")              --指明前导表                                                    INDEX_RS_ASC(@"SEL$1" "D"@"SEL$1" ("DEPT"."DEPTNO"))   --指明对于D上的访问方式为使用索引                               FULL(@"SEL$1" "E"@"SEL$1")                             --指明对于E上的访问方式为全表扫描                               OUTLINE_LEAF(@"SEL$1")                                                                                                 ALL_ROWS                                                                                                               OPTIMIZER_FEATURES_ENABLE('10.2.0.3')                                                                                  IGNORE_OPTIM_EMBEDDED_HINTS                                                                                            END_OUTLINE_DATA                                                                                                   */                                                                                                                                                                                                                                          Predicate Information (identified by operation id): --谓词信息部分，在执行计划中ID带有星号的每一行均对应到下面中的一行 ---------------------------------------------------                                                                                                                                                                                              2 - filter("E"."ENAME"='SMITH')                                                                                        4 - access("E"."DEPTNO"="D"."DEPTNO")                                                                                                                                                                                                      Column Projection Information (identified by operation id):  --执行时每一步骤所返回的列，下面的不同步骤返回了不同的列  -----------------------------------------------------------                                                                                                                                                                                      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 - "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]                                                                     3 - "D"."DEPTNO"[NUMBER,22], "D"."DNAME"[VARCHAR2,14], "D"."LOC"[VARCHAR2,13]                                          4 - "D".ROWID[ROWID,10], "D"."DEPTNO"[NUMBER,22]                                                                                                                                                                                           Note    --注释与描述部分，下面的描述中给出了本次SQL语句使用了动态采样功能                                              -----                                                                                                                     - dynamic sampling used for this statement                                                                                                                                                                                                 58 rows selected.

2、实际执行计划中的各字段与模块

SQL> select /*+ gather_plan_statistics */ *          --注意此处增加了提示gather_plan_statistics并且该语句被执行          2  from emp e,dept d                                                                                                   3  where e.deptno=d.deptno                                                                                             4  and e.ename='SMITH';                                                                                                                                                                                                                           7369 SMITH      CLERK           7902 17-DEC-80        800                    20         20 RESEARCH       DALLAS                                                                                                                        SQL> select * from table(dbms_xplan.display_cursor(null,null,'iostats last')); --使用display_cursor获取实际的执行计划                                                                                                                         SQL_ID  fpx7zw59f405d, child number 0              --这部分给出了SQL语句的SQL_ID,子游标号以及原始的SQL语句             -------------------------------------                                                                                  select /*+ gather_plan_statistics */ * from emp e,dept d where e.deptno=d.deptno and                                   e.ename='SMITH'                                                                                                                                                                                                                               Plan hash value: 351108634              --SQL 语句的哈希值                                                                                                     --SQL语句的执行计划，可以看到下面显示的字段一部分不同于预估执行计划中的字段    -----------------------------------------------------------------------------------------------------------            | Id  | Operation                    | Name    | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |            -----------------------------------------------------------------------------------------------------------            |   1 |  NESTED LOOPS                |         |      1 |      1 |      1 |00:00:00.01 |      10 |      1 |            |*  2 |   TABLE ACCESS FULL          | EMP     |      1 |      1 |      1 |00:00:00.01 |       8 |      0 |            |   3 |   TABLE ACCESS BY INDEX ROWID| DEPT    |      1 |      1 |      1 |00:00:00.01 |       2 |      1 |            |*  4 |    INDEX UNIQUE SCAN         | PK_DEPT |      1 |      1 |      1 |00:00:00.01 |       1 |      1 |            -----------------------------------------------------------------------------------------------------------                                                                                                                                   Predicate Information (identified by operation id):                                                                    ---------------------------------------------------                                                                                                                                                                                              2 - filter("E"."ENAME"='SMITH')                                                                                        4 - access("E"."DEPTNO"="D"."DEPTNO")                                                                                                                                                                                                      Note                                                                                                                   -----                                                                                                                     - dynamic sampling used for this statement                                                                                                                                                                                                                                                                                                                        26 rows selected.

三、总结  
由上可知，在不同的情形下可以获得执行计划的不同信息，而不同信息则展现了SQL语句对应的不同情况，因此应根据具体的情形具体分析。

[https_blog.csdn.net_jiangjunshow]: https://blog.csdn.net/jiangjunshow/article/details/77338485
[Link 1]: http://write.blog.csdn.net/postedit/6860007
[EXPLAIN PLAN _SQL]: http://blog.csdn.net/robinson_0612/article/details/6837771
[AUTOTRACE]: http://blog.csdn.net/robinson_0612/article/details/6030407
[dbms_xplan_display_cursor]: http://blog.csdn.net/robinson_0612/article/details/6866925