最近遇到了錯誤“Error: cannot fetch last explain plan from PLAN_TABLE”，於是稍微研究了一下哪些場景下碰到這種錯誤，具體參考下面案例：
1：忘記使用EXPLAIN PLAN放在SQL語句前面，然后使用使用SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY)查看具體SQL的執行計划時，就會遇到錯誤“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
其實，這種情形是因為SQL語句中忘記使用EXPLAIN PLAN，一般而言EXPLAIN PLAN會將SQL對應的執行計划放入plan_table。官方文檔介紹如下：
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.
如果沒有使用EXPLAIN PLAN，那么沒有將對應SQL的執行計划放進PLAN_TABLE，而如果使用EXPLAIN PLAN，那么ORACLE會用格式化的數據填充PLAN_TABLE表，以便以易讀的格式呈現給用戶。個人使用10046跟蹤對比了一下（對比使用EXPLAIN PLAN和不使用EXPLAIN PLAN兩種情況），使用EXPLAIN PLAN時，數據庫會向plan_table插入數據。如下所示：
2：對應的用戶下存在PLAN_TABLE表（這個可能情況比較復雜），然后使用ALTER SESSION SET CURRENT_SCHEMA設置當前會話的SCHEMA時可能會遇到這種場景。
在SCOTT用戶下創建一個PLAN_TABLE（結構一樣，如果結構不一樣，會報另外一種錯誤）
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用戶下不會出錯。
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>
但是我們使用ALTER SESSION SET CURRENT_SCHEMA設置當前會話的SCHEMA后，那么再按之前的SQL測試，就會遇到這個錯誤，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
如果遇到這種情況，可以使用上面腳本看看是否存在同名的PLAN_TABLE，這種情況下，可以將SCOTT下的PLAN_TABLE表重命名或刪除即可。當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>
當然，還可以更深入的探究，只是沒有太多價值，而且個人在測試過程中，發現還有許多其它狀況，例如解決了這個錯誤后，再去測試，就發現不報錯。但是顯示的執行計划還是原來SQL（不是當前SQL的執行計划）......... 。當然也不排除還有一些場景可能遇到這個錯誤。這里僅僅描述了兩種場景。