mysql 大表小表驱动的问题
有1个应用
select a.* , b.* from a, b
where a.id=b.id
说明 a是200万记录 b是700万记录
加入b是200万记录 a是700万记录
是否该语句一定要换为下面的
sql
select a.* , b.* from b，a
where b.id=a.id
（mysql 的驱动原则 小表带大表）
不需要，
mysql的优化器会自动实现的
刚好有两个表 t1, t_06,
t1 表 10000 记录， t_06 表 499999 记录
下面测试中，可以很明显看出，无论 t1, t_06 表顺序如何，MYSQL均会以同一方案执行。 另外也说明了在MySQL中不会象文件型数据库中那样有什么 a inner join b on a.id=b.id 比 from a,b where a.id=b.id 效率高之说。
-
SQL code
mysql> select count(*) from t1,t_06 where t1.id=t_06.id;
+----------+
| count(*) |
+----------+
| 100000 |
+----------+
1 row in set (1.89 sec)
mysql> select count(*) from t_06;
+----------+
| count(*) |
+----------+
| 499999 |
+----------+
1 row in set (0.02 sec)
mysql>
mysql> explain select * from t1,t_06 where t1.id=t_06.id;
+----+-------------+-------+--------+---------------+---------+---------+------------+--------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+------------+--------+-------+
| 1 | SIMPLE | t1 | ALL | PRIMARY | NULL | NULL | NULL | 100000 | |
| 1 | SIMPLE | t_06 | eq_ref | PRIMARY | PRIMARY | 4 | test.t1.id | 1 | |
+----+-------------+-------+--------+---------------+---------+---------+-------
-----+--------+-------+
2 rows in set (0.00 sec)
mysql> explain select * from t_06,t1 where t_06.id=t1.id;
+----+-------------+-------+--------+---------------+---------+---------+------------+--------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+------------+--------+-------+
| 1 | SIMPLE | t1 | ALL | PRIMARY | NULL | NULL | NULL | 100000 | |
| 1 | SIMPLE | t_06 | eq_ref | PRIMARY | PRIMARY | 4 | test.t1.id | 1 | |
+----+-------------+-------+--------+---------------+---------+---------+------------+--------+-------+
2 rows in set (0.00 sec)
mysql>
mysql> explain select * from t1 inner join t_06 using (id);
+----+-------------+-------+--------+---------------+---------+---------+------------+--------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+------------+--------+-------+
| 1 | SIMPLE | t1 | ALL | PRIMARY | NULL | NULL | NULL | 100000 | |
| 1 | SIMPLE | t_06 | eq_ref | PRIMARY | PRIMARY | 4 | test.t1.id | 1 | |
+----+-------------+-------+--------+---------------+---------+---------+------------+--------+-------+
2 rows in set (0.00 sec)
mysql> explain select * from t_06 inner join t1 using (id);
+----+-------------+-------+--------+---------------+---------+---------+------------+--------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+------------+--------+-------+
| 1 | SIMPLE | t1 | ALL | PRIMARY | NULL | NULL | NULL | 100000 | |
| 1 | SIMPLE | t_06 | eq_ref | PRIMARY | PRIMARY | 4 | test.t1.id | 1 | |
+----+-------------+-------+--------+---------------+---------+---------+------------+--------+-------+
2 rows in set (0.00 sec)
mysql>
