mysql - 简单JOIN时如何防止全表扫描？

Question

我有两个表，TableA 和 TableB:

CREATE TABLE `TableA` (
  `shared_id` int(10) unsigned NOT NULL default '0',
  `foo` int(10) unsigned NOT NULL,
  PRIMARY KEY  (`shared_id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1


CREATE TABLE `TableB` (
  `shared_id` int(10) unsigned NOT NULL auto_increment,
  `bar` int(10) unsigned NOT NULL,
  KEY `shared_id` (`shared_id`)
) ENGINE=MyISAM AUTO_INCREMENT=1001 DEFAULT CHARSET=latin1 

这是我的查询:

SELECT TableB.bar 
FROM TableB, TableA 
WHERE TableA.foo = 1000 
AND TableA.shared_id = TableB.shared_id;

问题是:

mysql> explain SELECT TableB.bar FROM TableB, TableA WHERE TableA.foo = 1000 AND TableA.shared_id = TableB.shared_id;

+----+-------------+--------------+--------+---------------+---------+---------+------------------------------------------+------+-------------+
| id | select_type | table        | type   | possible_keys | key     | key_len | ref                                      | rows | Extra       |
+----+-------------+--------------+--------+---------------+---------+---------+------------------------------------------+------+-------------+
|  1 | SIMPLE      | TableB       | ALL    | shared_id     | NULL    | NULL    | NULL                                     | 1000 |             |
|  1 | SIMPLE      | TableA       | eq_ref | PRIMARY       | PRIMARY | 4       | MyDatabase.TableB.shared_id              |    1 | Using where |
+----+-------------+--------------+--------+---------------+---------+---------+------------------------------------------+------+-------------+

我可以添加一个索引来防止对 TableB 进行全表扫描吗？

Answer 1

Runcible，您的查询可以使用一些重写。您应该始终在 ON 子句中而不是在 WHERE 中指定 JOIN 条件。

您的查询将变为:

SELECT TableB.bar 
FROM TableB
JOIN TableA
ON TableB.shared_id = TableA.shared_id
AND TableA.foo = 1000;

你不仅要这样做:

ALTER TABLE TableB ADD INDEX (shared_id,bar);

您需要按如下方式向 A 添加索引:

ALTER TABLE TableA ADD INDEX (foo, shared_id);

请执行此操作并提供 EXPLAIN 输出。

另请注意，通过在 (shared_id, bar) 上添加索引，您只是使 (shared_id) 索引变得多余。放下它。