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 ------------------------*/
rows函数的使用方法及实例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> ame,e.ade
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
------------------------------------- ame,e.ade 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 AND d.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 (identi
fied 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
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