EXPLAIN PLAN FOR 和 SET AUTOTRACE之间的差别

通常我们使用 EXPLAIN PLAN FOR 和 SET AUTOTRACE 来查看sql语句的执行计划，这里做一个小实验来看看两种方法对sql执行情况的差别。
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$sqlarea where sql_text like 'select * from yang_t';
EXECUTIONS PARSE_CALLS
---------- -----------
         1           1
yang@rac1>set autot on exp
yang@rac1> set autotrace  traceonly
yang@rac1>select * from yang_t;
22 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2508602004
----------------------------------------------------------------------------
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |        |    22 |  1738 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| YANG_T |    22 |  1738 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------
Note
-----
   - dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
         21  recursive calls
         61  db block gets
         33  consistent gets
          6  physical reads
      14040  redo size
       1082  bytes sent via SQL*Net to client
        520  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         22  rows processed
yang@rac1>set autot off
再次查询是否执行。可以看出使用set autotrace 查看执行计划时，oracle会执行一下sql语句的。
yang@rac1>select executions,parse_calls from v$sqlarea where sql_text like 'select * from yang_t';
EXECUTIONS PARSE_CALLS
---------- -----------
         2           2
对测试表进行dml操作。并查看dml 的执行情况。
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 ========
实验一下EXPLAIN PLAN 查看sql语句执行计划的情况。
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.
从下面的查询结果中可以看出使用EXPLAIN PLAN FOR 查看执行计划时oracle是没有执行要查看执行计划的sql 语句的。
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>
小结：
     EXPLAIN PLAN FOR 方式查看执行计划时oracle本身并不真正的执行该sql 语句，只是对sql进行解析获取执行计划。
    SET AUTOTRACE   方式查看sql语句的执行计划则是oracle 则对sql进行 解析并执行的。