在查看执行计划的时候，总是报错
模拟环境如下：
SQL> show user;
USER is "SCOTT"
SQL> select count(*) from emp;

  COUNT(*)
----------
        14

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------
SQL_ID  9babjv8yq8ru3, child number 0

BEGIN DBMS_OUTPUT.GET_LINES(:LINES, :NUMLINES); END;

NOTE: cannot fetch plan for SQL_ID: 9babjv8yq8ru3, CHILD_NUMBER: 0
      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)

8 rows selected.

最后发现是serveroutput惹的祸

SQL> show serveroutput
serveroutput ON SIZE UNLIMITED FORMAT WORD_WRAPPED
SQL> 
SQL> set serveroutput off
SQL> select count(*) from emp;

  COUNT(*)
----------
        14

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------
SQL_ID  g59vz2u4cu404, child number 0
-------------------------------------
select count(*) from emp

Plan hash value: 2937609675

-------------------------------------------------------------------------------------
| Id  | Operation        | Name   | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   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 |

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------

13 rows selected.