EXPLAIN PLAN FOR 和 SET AUTOTRACE之間的差別

通常我們使用 EXPLAIN PLAN FOR 和 SET AUTOTRACE 來檢視sql語句的執行計劃，這裡做一個小實驗來看看兩種方法對sql執行情況的差別。
yang@rac1>create table yang_t as select * from t;
Table created.
yang@rac1>set autot on exp
yang@rac1>select * from yang_t;
        ID  NAME
---------- ------------------
    130864 YANG_SEQ
    132031 YANG_A
    132032 SYS_C0066382
    132033 YANG_B
    132034 SYS_C0066383
    132035 FACT
    132036 MLOG$_YANG_A
    132037 MLOG$_YANG_B
    132038 MLOG$_FACT
    132039 T
    131949 YANG_ROWID
    131951 YANG_PK
    131952 SYS_C0066303
    131955 YANG_OBJECT
    131956 YANG_OID
    131957 SYS_C0066304
    132018 YANG_C
    132017 MV_CAPABILITIES_TABLE
    132030 MLOG$_YANG_PK
    132027 MLOG$_YANG_ROWID
           LINKORACL
           LINKYANG
22 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 2508602004
----------------------------------------------------------------------------
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |        |    22 |  1738 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| YANG_T |    22 |  1738 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------

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

yang@rac1>set autot off
yang@rac1>select executions,parse_calls from v$sqlarea where sql_text like 'select * from yang_t';
EXECUTIONS PARSE_CALLS
---------- -----------
         1           1
yang@rac1>set autot on exp
yang@rac1>set autotrace  traceonly
yang@rac1>select * from yang_t;
22 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2508602004
----------------------------------------------------------------------------
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |        |    22 |  1738 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| YANG_T |    22 |  1738 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------
Note
-----
   - dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
         21  recursive calls
         61  db block gets
         33  consistent gets
          6  physical reads
      14040  redo size
       1082  bytes sent via SQL*Net to client
        520  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         22  rows processed
yang@rac1>set autot off
再次查詢是否執行。可以看出使用set autotrace 檢視執行計劃時，oracle會執行一下sql語句的。
yang@rac1>select executions,parse_calls from v$sqlarea where sql_text like 'select * from yang_t';
EXECUTIONS PARSE_CALLS
---------- -----------
         2           2
對測試表進行dml操作。並檢視dml 的執行情況。
yang@rac1>insert into yang_t values (1,2);
1 row created.
yang@rac1>commit;
Commit complete.
yang@rac1>select executions,parse_calls from v$sqlarea where sql_text like 'insert into yang_t values (1,2);';
no rows selected
yang@rac1>select executions,parse_calls from v$sqlarea where sql_text like 'insert into yang_t values%';
EXECUTIONS PARSE_CALLS
---------- -----------
         1           1
=======EXPLAIN PLAN FOR ========
實驗一下EXPLAIN PLAN 檢視sql語句執行計劃的情況。
yang@rac1>EXPLAIN PLAN FOR SELECT * FROM YANG_T;
Explained.
yang@rac1>SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------
Plan hash value: 2508602004
----------------------------------------------------------------------------
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |        |    23 |  1817 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| YANG_T |    23 |  1817 |     4   (0)| 00:00:01 |
----------------------------------------------------------------------------
Note
-----
   - dynamic sampling used for this statement (level=2)
12 rows selected.
從下面的查詢結果中可以看出使用EXPLAIN PLAN FOR 檢視執行計劃時oracle是沒有執行要檢視執行計劃的sql 語句的。
yang@rac1>select executions,parse_calls from v$sqlarea where sql_text like 'SELECT * FROM YANG_T%';
no rows selected
yang@rac1>select executions,parse_calls from v$sqlarea where sql_text like 'SELECT * FROM YANG%';

no rows selected

yang@rac1>select executions,parse_calls from v$sqlarea where sql_text like 'select * from yang_t';
EXECUTIONS PARSE_CALLS
---------- -----------
         2           2
yang@rac1>
小結：
     EXPLAIN PLAN FOR 方式檢視執行計劃時oracle本身並不真正的執行該sql 語句，只是對sql進行解析獲取執行計劃。
    SET AUTOTRACE   方式檢視sql語句的執行計劃則是oracle 則對sql進行 解析並執行的。