MySql-straight_join优化列
STRAIGHT_JOIN 与 INNER JOIN 的功能完全一致
使用 INNER JOIN 时，mysql会根据优化规则自动判断 应该先加载哪个表
但有时自动的操作未必最优，就需要手动操作，其语法如下：
select ..from tab1 straiht_join tab2 where ...使用了 straight_join 后，tab1 会先于 tab2 载入。
【现象】
生产环境中遇到一个例子，执行sql需要1.29s 已经超出业务方的要求，需要进行优化，sql 如下
select d.instance_no,d.zone_id, d.region_no,d.user_id,d.cores,d.mem,d.disk, d.tx_pub , u.idkp , m.image_no, m.platform, m.image_size
from user u, instance d , image m
where d.region_no = 'cn-cm9002' and
u.user_id = d.user_id and
d.image_id = m.image_id and
d.status != 8 and
d.gmt_create <'2013-08-19 14:00:00';
-------------------------------
3120 rows in set （1.29 sec）
hy@3309 03:09:09>explain select d.instance_no,d.zone_id, d.region_no,d.user_id,d.cores,d.mem,d.disk, d.tx_pub , u.idkp , m.image_no, m.platform, m.image_size
-> from user u, instance d , image m
-> where d.region_no = 'cn-cm9002' and
-> u.user_id = d.user_id and
-> d.image_id = m.image_id and
-> d.status != 8 and
-> d.gmt_create <'2013-08-19 14:00:00';
+----+-------------+-------+--------+-------------------------------------+--------------------+---------+------------------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+-------------------------------------+--------------------+---------+------------------+--------+-------------+
| 1 | SIMPLE | u | index | PRIMARY | idkp | 98 | NULL | 133002 | Using index |
| 1 | SIMPLE | d | ref | image_id,ind_i_uid_hostname,user_id | ind_i_uid_hostname | 4 | hy.u.user_id | 1 | Using where |
| 1 | SIMPLE | m | eq_ref | PRIMARY | PRIMARY | 4 | hy.d.image_id | 1 | |
+----+-------------+-------+--------+-------------------------------------+--------------------+---------+------------------+--------+-------------+
3 rows in set (0.00 sec)【解决方法】
使用 straight_join 方式优化sql 执行的顺序 结果如下：
rac1@3309 15:01:55>explain select d.instance_no,d.zone_id, d.region_no,d.user_id,d.cores,d.mem,d.disk, d.tx_pub , u.idkp , m.image_no, m.platform, m.image_size
-> from instance d straight_join user u on u.user_id = d.user_id, image m
-> where d.region_no = 'cn-cm9002' and
-> d.image_id = m.image_id and
-> d.status != 8 and
-> d.gmt_create <'2013-08-19 14:00:00';
+----+-------------+-------+--------+-------------------------------------+---------+---------+------------------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+-------------------------------------+---------+---------+------------------+--------+-------------+
| 1 | SIMPLE | d | ALL | image_id,ind_i_uid_hostname,user_id | NULL | NULL | NULL | 316473 | Using where |
| 1 | SIMPLE | m | eq_ref | PRIMARY | PRIMARY | 4 | hy.d.image_id | 1 | |
| 1 | SIMPLE | u | eq_ref | PRIMARY | PRIMARY | 4 | hy.d.user_id | 1 | |
+----+-------------+-------+--------+-------------------------------------+---------+---------+------------------+--------+-------------+
3 rows in set (0.00 sec)
--------------------------------
3120 rows in set (0.39 sec)【问题分析】
上面的介绍中描述mysql的优化器只支持 nest loop ，对于多表连接会mysql优化器采用了简单的方式：选择结果集小的表作为驱动表。
instance表连接 user表有两种连接方式：
A 选择user 表作为驱动表 优化器扫描133002行
B 选择instance表作为驱动表 优化器扫描 316473行
因此优化器选择了看起来正确的执行计划 以user表作为驱动表。但是我们查看where条件，正确的应该是通过instance 的region_no，status ，gmt_create 过滤得到instance的结果集，再来和user，image中的表进行关联。
而执行计划是扫描user表中全部的记录再去关联instance 表和image表，显然执行顺序有偏差。因此加上straight_join hint之后，强制优化器选择 instance为驱动表，按照正确的执行计划执行。
附上表的记录数：
hy@3309 01:11:17> select count(*) from user; +----------+
| count(*) | +----------+
| 134221 | +----------+
1 row in set (0.02 sec)
hy@3309 01:19:44> select count(*) from instance; +----------+
| count(*) | +----------+
| 375732 | +----------+
1 row in set (0.06 sec)
hy@3309 01:19:54> select count(*) from image; +----------+
| count(*) | +----------+
| 18858 | +----------+
1 row in set (0.00 sec)