cannot fetch plan for SQL_ID

  SQL tuning过程中离不开分析SQL语句的执行计划。在一次提取执行计划的时候碰到cannot fetch plan for SQL_ID的错误提示。根据错误
提示来看需要检查SQL的子游标或该执行计划不在v$sql_plan表中，而这种情况一般不存在。因为刚刚执行过的SQL语句不可能这么快从v$sql_plan
移除。下面给出错误描述及处理办法。

1、故障现象
-->使用display_cursor提取执行计划失败
  admin@CADB> select * from table(dbms_xplan.display_cursor(null,null,'iostats last'));
  
  PLAN_TABLE_OUTPUT
  ---------------------------------------------------------------------------------------------------
  SQL_ID  9babjv8yq8ru3, child number 3
  
  BEGIN DBMS_OUTPUT.GET_LINES(:LINES, :NUMLINES); END;
  
  NOTE: cannot fetch plan for SQL_ID: 9babjv8yq8ru3, CHILD_NUMBER: 3
        Please verify value of SQL_ID and CHILD_NUMBER;
        It could also be that the plan is no longer in cursor cache (check v$sql_plan)

-->查看serveroutput设置值，此时为ON，再次调用dbms_xplan.display_cursor，此时故障同上
  scott@CADB> show serveroutput
  serveroutput ON SIZE 1000000 FORMAT WORD_WRAPPED
  scott@CADB> select /*+ gather_plan_statistics */ * from dept;
  
      DEPTNO DNAME          LOC
  ---------- -------------- -------------
          10 ACCOUNTING     NEW YORK
          20 RESEARCH       DALLAS
          30 SALES          CHICAGO
          40 OPERATIONS     BOSTON
          50 DEVELOPE       CHINA
  
  scott@CADB> select * from table(dbms_xplan.display_cursor(null,null,'iostats last'));
  
  PLAN_TABLE_OUTPUT
  ----------------------------------------------------------------------------------------------
  SQL_ID  9babjv8yq8ru3, child number 2
  
  BEGIN DBMS_OUTPUT.GET_LINES(:LINES, :NUMLINES); END;
  
  NOTE: cannot fetch plan for SQL_ID: 9babjv8yq8ru3, CHILD_NUMBER: 2
        Please verify value of SQL_ID and CHILD_NUMBER;
        It could also be that the plan is no longer in cursor cache (check v$sql_plan)

2、解决故障
-->关闭serveroutput选项
  scott@CADB> set serveroutput off;
  scott@CADB> select /*+ gather_plan_statistics */ * from dept;
  
      DEPTNO DNAME          LOC
  ---------- -------------- -------------
          10 ACCOUNTING     NEW YORK
          20 RESEARCH       DALLAS
          30 SALES          CHICAGO
          40 OPERATIONS     BOSTON
          50 DEVELOPE       CHINA

  /**************************************************/
  /* Author: Robinson Cheng                         */
  /* Blog:   http://blog.csdn.net/robinson_0612     */
  /* MSN:    [email protected]              */
  /* QQ:     645746311                              */
  /**************************************************/
  
-->此时成功提取执行计划
  scott@CADB> select * from table(dbms_xplan.display_cursor(null,null,'iostats last'));
  
  PLAN_TABLE_OUTPUT
  ------------------------------------------------------------------------------------------------
  SQL_ID  08u3pfapxj6g5, child number 0
  -------------------------------------
  select /*+ gather_plan_statistics */ * from dept
  
  Plan hash value: 3383998547
  
  ------------------------------------------------------------------------------------
  | Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
  ------------------------------------------------------------------------------------
  |   1 |  TABLE ACCESS FULL| DEPT |      1 |      5 |      5 |00:00:00.01 |       8 |
  ------------------------------------------------------------------------------------
  
  12 rows selected.  

-->下面使用explain plan加载执行计划到plan_table，然后使用dbms_xplan.display提取执行计划
-->从下面的实现中可知，serveroutput不影响从plan_table提取执行计划
  scott@CADB> show serveroutput
  serveroutput ON SIZE 1000000 FORMAT WORD_WRAPPED
  scott@CADB> explain plan for select count(*) from emp;
  
  Explained.
  
  scott@CADB> select * from table(dbms_xplan.display);    
  
  PLAN_TABLE_OUTPUT
  -----------------------------------------------------------------------------------------------------------------------------------
  Plan hash value: 1858788047
  
  -------------------------------------------------------------------------
  | Id  | Operation        | Name         | Rows  | Cost (%CPU)| Time     |
  -------------------------------------------------------------------------
  |   0 | SELECT STATEMENT |              |     1 |     1   (0)| 00:00:01 |
  |   1 |  SORT AGGREGATE  |              |     1 |            |          |
  |   2 |   INDEX FULL SCAN| I_EMP_DEPTNO |    14 |     1   (0)| 00:00:01 |
  -------------------------------------------------------------------------
  
  9 rows selected.

-->很多时候DBA在login.sql 或 glogin.sql 时指定了serveroutput为ON，以便在执行PL/SQL时能正常输出，而在获取SQL的执行计划时，恰恰
-->由于该设置而导致无法提取执行计划。不过serveroutput不影响从plan_table提取执行计划。

和执行计划相关链接：

dbms_xplan之display_cursor函数的使用

dbms_xplan之display函数的使用

执行计划中各字段各模块描述

使用 EXPLAIN PLAN 获取SQL语句执行计划 

启用 AUTOTRACE 功能