Error: cannot fetch last explain plan from PLAN_TABLE

最近遇到了错误“Error: cannot fetch last explain plan from PLAN_TABLE”，于是稍微研究了一下哪些场景下碰到这种错误，具体参考下面案例：

 

 

1：忘记使用EXPLAIN PLAN放在SQL语句前面，然后使用使用SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY)查看具体SQL的执行计划时，就会遇到错误“Error: cannot fetch last explain plan from PLAN_TABLE”。如下所示：

 

SQL> show user;
USER is "SYS"
SQL> SELECT *
  2  FROM SCOTT.EMP
  3  WHERE HIREDATE BETWEEN '01-JAN-1981' AND '01-APR-1981';

     EMPNO ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
      7499 ALLEN      SALESMAN        7698 20-FEB-81       1600        300         30
      7521 WARD       SALESMAN        7698 22-FEB-81       1250        500         30

SQL> COL PLAN_TABLE_OUTPUT FOR A180;
SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------
Error: cannot fetch last explain plan from PLAN_TABLE

 

 

其实，这种情形是因为SQL语句中忘记使用EXPLAIN PLAN，一般而言EXPLAIN PLAN会将SQL对应的执行计划放入plan_table。官方文档介绍如下：

 

The EXPLAIN PLAN statement displays execution plans chosen by the Oracle optimizer for SELECT, UPDATE, INSERT, and DELETE statements. A statement's execution plan is the sequence of operations Oracle performs to run the statement. The row source tree is the core of the execution plan.

 

如果没有使用EXPLAIN PLAN，那么没有将对应SQL的执行计划放进PLAN_TABLE，而如果使用EXPLAIN PLAN，那么ORACLE会用格式化的数据填充PLAN_TABLE表，以便以易读的格式呈现给用户。个人使用10046跟踪对比了一下（对比使用EXPLAIN PLAN和不使用EXPLAIN PLAN两种情况），使用EXPLAIN PLAN时，数据库会向plan_table插入数据。如下所示：

 

 

2：对应的用户下存在PLAN_TABLE表（这个可能情况比较复杂），然后使用ALTER SESSION SET CURRENT_SCHEMA设置当前会话的SCHEMA时可能会遇到这种场景。

 

在SCOTT用户下创建一个PLAN_TABLE（结构一样，如果结构不一样，会报另外一种错误）

 

 

SQL> SHOW USER;
USER is "SCOTT"
SQL>CREATE TABLE PLAN_TABLE AS
  SELECT STATEMENT_ID,
         PLAN_ID,
         TIMESTAMP,
         REMARKS,
         OPERATION,
         OPTIONS,
         OBJECT_NODE,
         OBJECT_OWNER,
         OBJECT_NAME,
         OBJECT_ALIAS,
         OBJECT_INSTANCE,
         OBJECT_TYPE,
         OPTIMIZER,
         SEARCH_COLUMNS,
         ID,
         PARENT_ID,
         DEPTH,
         POSITION,
         COST,
         CARDINALITY,
         BYTES,
         OTHER_TAG,
         PARTITION_START,
         PARTITION_STOP,
         PARTITION_ID,
         TO_LOB(OTHER) AS OTHER,
         OTHER_XML     AS OTHER_XML,
         DISTRIBUTION,
         CPU_COST,
         IO_COST,
         TEMP_SPACE,
         ACCESS_PREDICATES,
         FILTER_PREDICATES,
         PROJECTION,
         TIME,
         QBLOCK_NAME
  FROM   PLAN_TABLE;
SQL> EXPLAIN PLAN FOR
  2  SELECT * FROM DUAL;

Explained.

SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY); #SCOTT用户下不会出错。

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------
Plan hash value: 272002086

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |     2 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| DUAL |     1 |     2 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------
8 rows selected.

SQL>

 

但是我们使用ALTER SESSION SET CURRENT_SCHEMA设置当前会话的SCHEMA后，那么再按之前的SQL测试，就会遇到这个错误，如下所示：

 

SQL> show user;
USER is "SYS"
SQL> alter session set current_schema=SCOTT;

Session altered.

SQL> EXPLAIN PLAN FOR
  2  SELECT *
  3  FROM SCOTT.EMP
WHERE HIREDATE BETWEEN '01-JAN-1981' AND '01-APR-1981';  4

Explained.

SQL> COL PLAN_TABLE_OUTPUT FOR A180;
SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Error: cannot fetch last explain plan from PLAN_TABLE


SQL> SET LINESIZE 1200
SQL> SELECT OWNER,OBJECT_NAME,OBJECT_TYPE,CREATED FROM ALL_OBJECTS
  2  WHERE OBJECT_NAME LIKE 'PLAN_TABLE%'
  3  AND OWNER IN (SYS_CONTEXT('USERENV','CURRENT_SCHEMA'),'PUBLIC','SYS');

OWNER                          OBJECT_NAME                    OBJECT_TYPE         CREATED
------------------------------ ------------------------------ ------------------- ---------
SYS                            PLAN_TABLE$                    TABLE               24-MAY-15
PUBLIC                         PLAN_TABLE                     SYNONYM             30-JUN-05
SCOTT                          PLAN_TABLE                     TABLE               21-DEC-19

 

 

 如果遇到这种情况，可以使用上面脚本看看是否存在同名的PLAN_TABLE，这种情况下，可以将SCOTT下的PLAN_TABLE表重命名或删除即可。当然也可以用下面方法

 

SQL> EXPLAIN PLAN INTO SCOTT.PLAN_TABLE FOR
  2  SELECT *
  3  FROM SCOTT.EMP
WHERE HIREDATE BETWEEN '01-JAN-1981' AND '01-APR-1981';
Explained.

SQL> COL PLAN_TABLE_OUTPUT FOR A180;
SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     2 |    74 |     2   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| EMP  |     2 |    74 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------
   1 - filter("HIREDATE"<=TO_DATE(' 1981-04-01 00:00:00', 'syyyy-mm-dd
              hh24:mi:ss') AND "HIREDATE">=TO_DATE(' 1981-01-01 00:00:00',
              'syyyy-mm-dd hh24:mi:ss'))

15 rows selected.

SQL>

当然，还可以更深入的探究，只是没有太多价值，而且个人在测试过程中，发现还有许多其它状况，例如解决了这个错误后，再去测试，就发现不报错。但是显示的执行计划还是原来SQL（不是当前SQL的执行计划）......... 。当然也不排除还有一些场景可能遇到这个错误。这里仅仅描述了两种场景。