mysql sql执行顺序

<pre name="code" class="html">mysql>  explain select * from (select * from ( select * from test where id=1) a) b;
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
|  1 | PRIMARY     | <derived2> | ALL  | NULL          | NULL | NULL    | NULL |    2 | NULL        |
|  2 | DERIVED     | <derived3> | ALL  | NULL          | NULL | NULL    | NULL |    2 | NULL        |
|  3 | DERIVED     | test       | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using where |
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
3 rows in set (0.00 sec)

很明显 这里ID=3 先执行, ID=2 select_type=DERIVED是一个派生表指向<derived3> 表示3产生的派生表

ID=1 PRIMARY:最外面的SELECT   最外面执行的SELECT <derived2> 表示ID=2产生的派生表

mysql> explain select t1.* from t2 ,t1 where t2.id=t1.id;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                              |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |    2 | NULL                                               |
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
2 rows in set (0.00 sec)

如果id相同，则执行顺序从上至下。

SIMPLE
简单SELECT(不使用UNION或子查询等)

id=1 t2返回的记录 t2为驱动表,传递给t1

对比Oracle的嵌套循环:

SQL> explain plan for select t1.* from t1,t2 where t1.id=t2.id;

已解释。

SQL> select * from table(dbms.exlain.display());
select * from table(dbms.exlain.display())
                    *
第 1 行出现错误:
ORA-00904: "DBMS"."EXLAIN"."DISPLAY": 标识符无效

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 2959412835

---------------------------------------------------------------------------
| Id  | Operation	   | Name | Rows  | Bytes | Cost (%CPU)| Time	  |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	  |	7 |   266 |	6   (0)| 00:00:01 |
|*  1 |  HASH JOIN	   |	  |	7 |   266 |	6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T2   |	6 |    78 |	3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| T1   |	7 |   175 |	3   (0)| 00:00:01 |
---------------------------------------------------------------------------

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("T1"."ID"="T2"."ID")

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

已选择19行

采集统计信息:

BEGIN
  DBMS_STATS.GATHER_TABLE_STATS(ownname          => 'QUERY',
                                tabname          => 'T1',
                                estimate_percent => 30,
                                method_opt       => 'for all columns size repeat',
                                no_invalidate    => FALSE,
                                degree           => 8,
                                cascade          => TRUE);
END;

BEGIN
  DBMS_STATS.GATHER_TABLE_STATS(ownname          => 'QUERY',
                                tabname          => 'T2',
                                estimate_percent => 30,
                                method_opt       => 'for all columns size repeat',
                                no_invalidate    => FALSE,
                                degree           => 8,
                                cascade          => TRUE);
END;

SQL> explain plan for select t1.* from t1,t2 where t1.id=t2.id;

已解释。

SQL>  select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 469473159

------------------------------------------------------------------------------
| Id  | Operation	   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	     |	   7 |	 119 |	   3   (0)| 00:00:01 |
|   1 |  NESTED LOOPS	   |	     |	   7 |	 119 |	   3   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T1      |	   7 |	  98 |	   3   (0)| 00:00:01 |
|*  3 |   INDEX RANGE SCAN | T2_IDX1 |	   1 |	   3 |	   0   (0)| 00:00:01 |
------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("T1"."ID"="T2"."ID")

已选择15行。

mysql> explain select t1.* from t2 ,t1 where t2.id=t1.id;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                              |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |    2 | NULL                                               |
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------------------------------------------+
2 rows in set (0.00 sec)

如果id相同，则执行顺序从上至下。

SIMPLE
简单SELECT(不使用UNION或子查询等)

id=1 t2返回的记录 t2为驱动表,传递给t1

原理类似: