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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memstatsrun_stats_last 等同于iostats last。只能用于oracle 10g R1run_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 ANDd.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 (转载请注明出处：[url=http://www.tdtf.org]易博网[/url] [url=http://www.k5048.com]博彩网[/url])
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