案例：使用dbms_xplan.display_cursor无法获取执行计划

test@DEMO> select count(*) from t;

  COUNT(*)
----------
     86391

test@DEMO> @x

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID  9babjv8yq8ru3, child number 1

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

NOTE: cannot fetch plan for SQL_ID: 9babjv8yq8ru3, CHILD_NUMBER: 1
      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.

test@DEMO> get x 
  1* select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

test@DEMO> show serveroutput
serveroutput ON SIZE 1000000 FORMAT WORD_WRAPPED
test@DEMO> set serveroutput off
test@DEMO> show serveroutput
serveroutput OFF

test@DEMO> select count(*) from t;

  COUNT(*)
----------
     86391

test@DEMO> @x

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------
SQL_ID  cyzznbykb509s, child number 1
-------------------------------------
select count(*) from t

Plan hash value: 2966233522

-------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |      1 |00:00:00.05 |    1239 |
|   1 |  SORT AGGREGATE    |      |      1 |      1 |      1 |00:00:00.05 |    1239 |
|   2 |   TABLE ACCESS FULL| T    |      1 |  73056 |  86391 |00:00:00.03 |    1239 |
-------------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


18 rows selected.

cd $ORACLE_HOME/sqlplus/admin
vi glogin.sql 
...省略无关内容...
set serveroutput on size 1000000
...省略无关内容...