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，具體參考下麵案例： 1：忘記使用EXPLAIN PLAN放在SQL語句前面，然後使用使用SELECT * FROM TABLE(DB... ...
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，具體參考下麵案例：
1：忘記使用EXPLAIN PLAN放在SQL語句前面，然後使用使用SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY)查看具體SQL的執行計劃時，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.EMP3 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 307521 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對應的執行計劃放入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，那麼沒有將對應SQL的執行計劃放進PLAN_TABLE，而如果使用EXPLAIN PLAN，那麼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，資料庫會向plan_table插入數據。如下所示：
2：對應的用戶下存在PLAN_TABLE表（這個可能情況比較複雜），然後使用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後，那麼再按之前的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.EMPWHERE 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_OU
