根据 MySQL 文档，如果最左边的字段是标准的一部分，则仍将使用复合索引。但是，此表将无法与主键正确连接；我必须添加左侧两个字段的另一个索引，然后使用。
其中一个表是内存，我知道默认情况下内存使用不能用于组/订单的哈希索引。但是我使用的是内存表的所有行而不是索引，所以我认为这与问题无关。
我错过了什么？
mysql> show create table pr_temp;
| pr_temp | CREATE TEMPORARY TABLE `pr_temp` (
`player_id` int(10) unsigned NOT NULL,
`insert_date` date NOT NULL,
[...]
PRIMARY KEY (`player_id`,`insert_date`) USING BTREE,
KEY `insert_date` (`insert_date`)
) ENGINE=MEMORY DEFAULT CHARSET=utf8 |
mysql> show create table player_game_record;
| player_tank_record | CREATE TABLE `player_game_record` (
`player_id` int(10) unsigned NOT NULL,
`game_id` smallint(5) unsigned NOT NULL,
`insert_date` date NOT NULL,
[...]
PRIMARY KEY (`player_id`,`insert_date`,`game_id`),
KEY `insert_date` (`insert_date`),
KEY `player_date` (`player_id`,`insert_date`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 DATA DIRECTORY='...' INDEX DIRECTORY='...' |
mysql> explain select pgr.* from player_game_record pgr inner join pr_temp on pgr.player_id = pr_temp.player_id and pgr.insert_date = pr_temp.date_prev;
+----+-------------+---------+------+---------------------------------+-------------+---------+-------------------------------------------------------------------------+--------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+---------------------------------+-------------+---------+-------------------------------------------------------------------------+--------+-------+
| 1 | SIMPLE | pr_temp | ALL | PRIMARY | NULL | NULL | NULL | 174683 | |
| 1 | SIMPLE | pgr | ref | PRIMARY,insert_date,player_date | player_date | 7 | test_gamedb.pr_temp.player_id,test_gamedb.pr_temp.date_prev | 21 | |
+----+-------------+---------+------+---------------------------------+-------------+---------+-------------------------------------------------------------------------+--------+-------+
2 rows in set (0.00 sec)
mysql> explain select pgr.* from player_game_record pgr force index (primary) inner join pr_temp on pgr.player_id = pr_temp.player_id and pgr.insert_date = pr_temp.date_prev;
+----+-------------+---------+------+---------------+---------+---------+-------------------------------------------------------------------------+---------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+---------------+---------+---------+-------------------------------------------------------------------------+---------+-------+
| 1 | SIMPLE | pr_temp | ALL | PRIMARY | NULL | NULL | NULL | 174683 | |
| 1 | SIMPLE | pgr | ref | PRIMARY | PRIMARY | 7 | test_gamedb.pr_temp.player_id,test_gamedb.pr_temp.date_prev | 2873031 | |
+----+-------------+---------+------+---------------+---------+---------+-------------------------------------------------------------------------+---------+-------+
2 rows in set (0.00 sec)
我认为主键应该可以工作，使用左两列（player_id，insert_date）。但是，默认情况下它将使用 player_date 索引，如果我强制它使用主索引，它看起来只使用一个字段而不是两个字段。
Update2：Mysql version 5.5.27-log Update3：（注意这是在尝试其他一些测试时删除 player_date 索引之后）
mysql> show indexes in player_game_record;
+--------------------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------------------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| player_game_record | 0 | PRIMARY | 1 | player_id | A | NULL | NULL | NULL | | BTREE | | |
| player_game_record | 0 | PRIMARY | 2 | insert_date | A | NULL | NULL | NULL | | BTREE | | |
| player_game_record | 0 | PRIMARY | 3 | game_id | A | 576276246 | NULL | NULL | | BTREE | | |
| player_game_record | 1 | insert_date | 1 | insert_date | A | 33304 | NULL | NULL | | BTREE | | |
+--------------------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (1.08 sec)
mysql> select count(*) from player_game_record;
+-----------+
| count(*) |
+-----------+
| 576276246 |
+-----------+
1 row in set (0.00 sec)