mysql - 对于关联表的查询使用join或where进行关联.具体的效率有没有很大区别?

Question

两张表.manytoone的关系.查询many,通过外键随便把one端得信息查出.我们可以通过left outer join on鞥等的join查询进行抓取.或者可以通过 where many端得外键id=one端得主键 效果是一样的.那他们具体的查询效率是否有区别?

Answer 1

看情况，如果你的devicespec1.id是主键或者索引什么的，那么join效率高。看这个就清楚了：

mysql> explain select test_tb1.id, test_tb2.value from test_tb1, test_tb2 where test_tb1.id = test_tb2.id;
+----+-------------+----------+-------+---------------+---------+---------+------+------+--------------------------------+
| id | select_type | table    | type  | possible_keys | key     | key_len | ref  | rows | Extra                          |
+----+-------------+----------+-------+---------------+---------+---------+------+------+--------------------------------+
|  1 | SIMPLE      | test_tb1 | index | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using index                    |
|  1 | SIMPLE      | test_tb2 | ALL   | PRIMARY       | NULL    | NULL    | NULL |    4 | Using where; Using join buffer |
+----+-------------+----------+-------+---------------+---------+---------+------+------+--------------------------------+
2 rows in set (0.01 sec)

mysql> explain select test_tb1.id, test_tb2.value from test_tb1 left join test_tb2 on test_tb1.id = test_tb2.id;
+----+-------------+----------+--------+---------------+---------+---------+------------------+------+-------------+
| id | select_type | table    | type   | possible_keys | key     | key_len | ref              | rows | Extra       |
+----+-------------+----------+--------+---------------+---------+---------+------------------+------+-------------+
|  1 | SIMPLE      | test_tb1 | index  | NULL          | PRIMARY | 4       | NULL             |    3 | Using index |
|  1 | SIMPLE      | test_tb2 | eq_ref | PRIMARY       | PRIMARY | 4       | test.test_tb1.id |    1 |             |
+----+-------------+----------+--------+---------------+---------+---------+------------------+------+-------------+
2 rows in set (0.00 sec)

第一次查询要遍历两张表，第二次查询使用eq_ref不用遍历第二张表

Answer 2

楼上的答案不靠谱. 1. 楼主明确说了,这里用外键关联, 实验里应该没有; 2. 只用了区区3,4条数据来做实验.

做了实验, mysql5.5.24 两张表每张10w数据. "where进行关联" 实际上被转为join. 实验如下:

mysql> show create table x;
+-------+---------------------------------------------------------------------------------------------------------+
| Table | Create Table    |
+-------+---------------------------------------------------------------------------------------------------------+
| x     | CREATE TABLE `x` (
  `id` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+---------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show create table y;
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table             |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| y     | CREATE TABLE `y` (
  `id` int(11) NOT NULL,
  `xid` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_y_x` (`xid`),
  CONSTRAINT `fk_y_x` FOREIGN KEY (`xid`) REFERENCES `x` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.03 sec)

mysql> select count(*) from x;
+----------+
| count(*) |
+----------+
|   100000 |
+----------+
1 row in set (0.04 sec)

mysql> select count(*) from y;
+----------+
| count(*) |
+----------+
|   100000 |
+----------+
1 row in set (0.04 sec)

mysql> explain extended select x.id, y.id from x,y where x.id=y.xid;
+----+-------------+-------+-------+---------------+---------+---------+------------+-------+----------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref        | rows  | filtered | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------------+-------+----------+--------------------------+
|  1 | SIMPLE      | x     | index | PRIMARY       | PRIMARY | 4       | NULL       | 95012 |   100.00 | Using index              |
|  1 | SIMPLE      | y     | ref   | fk_y_x        | fk_y_x  | 5       | test1.x.id |     1 |   100.00 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------+---------+------------+-------+----------+--------------------------+
2 rows in set, 1 warning (0.00 sec)

mysql> show warnings;
+-------+------+-----------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                           |
+-------+------+-----------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | select `test1`.`x`.`id` AS `id`,`test1`.`y`.`id` AS `id` from `test1`.`x` join `test1`.`y` where (`test1`.`y`.`xid` = `test1`.`x`.`id`) |
+-------+------+-----------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> explain extended select x.id, y.id from x left join y on x.id=y.xid;
+----+-------------+-------+-------+---------------+---------+---------+------------+-------+----------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref        | rows  | filtered | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------------+-------+----------+-------------+
|  1 | SIMPLE      | x     | index | NULL          | PRIMARY | 4       | NULL       | 95012 |   100.00 | Using index |
|  1 | SIMPLE      | y     | ref   | fk_y_x        | fk_y_x  | 5       | test1.x.id |     1 |   100.00 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+------------+-------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

mysql> show warnings;
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                      |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | select `test1`.`x`.`id` AS `id`,`test1`.`y`.`id` AS `id` from `test1`.`x` left join `test1`.`y` on((`test1`.`x`.`id` = `test1`.`y`.`xid`)) where 1 |
+-------+------+----------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

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