一个表中的两个索引，是仅在where子句中使用的第二个索引值得吗？

Question

比方说，我有以下MySQL表：

CREATE TABLE player (
    id int(9) unsigned NOT NULL DEFAULT 0 , 
    score MEDIUMINT(8) unsigned NOT NULL DEFAULT 0, 
    signupdate DATE NOT NULL, 
    lastupdate DATE NOT NULL 
) DEFAULT CHARACTER SET utf8 ENGINE=InnoDB; 

目前我有id列主键。 lastupdate列每天更新一次，如果未更新则表示玩家已删除该帐户，这意味着该列的可卡因性非常低。 也有一个关系表与精密组件相匹配matchid，playerid和matchdate

我的大部分查询都像

SELECT id,score,signupdate FROM player 
JOIN matches ON matches.playerid = player.id 
WHERE lastupdate = '{today}' 

所以3箱指数来我的心

1. 的ID PRIMARY KEY
2. ID和主键在lastupdate上的主键
3. PRIMARY KEY on（id，lastupdate）

哪一个会是最好的？

Answer 1

您应该在表matches列playerid上的索引以及表player列lastupdate上的索引。

作为一个非常粗略的经验法则，如果它是一个大型表，那么在WHERE和JOIN子句中使用的应该有一个索引。

要获得更多信息，可以使用explain声明。这是它的样子。注意explain声明在最后：

mysql> CREATE TABLE player (
    ->  id int(9) unsigned NOT NULL DEFAULT 0 , 
    ->  score MEDIUMINT(8) unsigned NOT NULL DEFAULT 0, 
    ->  signupdate DATE NOT NULL, 
    ->  lastupdate DATE NOT NULL 
    ->) DEFAULT CHARACTER SET utf8 ENGINE=InnoDB; 
Query OK, 0 rows affected (0.12 sec) 

mysql> 
mysql> CREATE TABLE matches (
    ->  matchid int(9) unsigned NOT NULL DEFAULT 0 , 
    ->  playerid int(9) unsigned NOT NULL DEFAULT 0 , 
    ->  matchdate DATE NOT NULL 
    ->) DEFAULT CHARACTER SET utf8 ENGINE=InnoDB; 
Query OK, 0 rows affected (0.22 sec) 

mysql> 
mysql> SELECT id,score,signupdate FROM player 
    -> JOIN matches ON matches.playerid = player.id 
    -> WHERE lastupdate = now() 
    -> ; 
Empty set (0.00 sec) 

mysql> 
mysql> explain 
    -> SELECT id,score,signupdate FROM player 
    -> JOIN matches ON matches.playerid = player.id 
    -> WHERE lastupdate = '{today}' 
    -> ; 
+----+-------------+---------+------+---------------+------+---------+------+------+--------------------------------+ 
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra       | 
+----+-------------+---------+------+---------------+------+---------+------+------+--------------------------------+ 
| 1 | SIMPLE  | player | ALL | NULL   | NULL | NULL | NULL | 1 | Using where     | 
| 1 | SIMPLE  | matches | ALL | NULL   | NULL | NULL | NULL | 1 | Using where; Using join buffer | 
+----+-------------+---------+------+---------------+------+---------+------+------+--------------------------------+ 
2 rows in set, 2 warnings (0.00 sec) 

mysql> CREATE INDEX player_idx_1 
    -> ON player (id) 
    -> ; 
Query OK, 0 rows affected (0.15 sec) 
Records: 0 Duplicates: 0 Warnings: 0 

mysql> CREATE INDEX matches_idx_1 
    -> ON matches (playerid) 
    -> ; 
Query OK, 0 rows affected (0.16 sec) 
Records: 0 Duplicates: 0 Warnings: 0 

mysql> 
mysql> explain SELECT id,score,signupdate FROM player JOIN matches ON matches.playerid = player.id WHERE lastupdate = '{today}'; 
+----+-------------+---------+------+---------------+---------------+---------+-----------------+------+-------------+ 
| id | select_type | table | type | possible_keys | key   | key_len | ref    | rows | Extra  | 
+----+-------------+---------+------+---------------+---------------+---------+-----------------+------+-------------+ 
| 1 | SIMPLE  | player | ALL | player_idx_1 | NULL   | NULL | NULL   | 1 | Using where | 
| 1 | SIMPLE  | matches | ref | matches_idx_1 | matches_idx_1 | 4  | mysql.player.id | 1 | Using index | 
+----+-------------+---------+------+---------------+---------------+---------+-----------------+------+-------------+ 
2 rows in set, 2 warnings (0.00 sec) 

mysql> 

定义添加索引lastupdate

mysql> CREATE INDEX player_idx_2 
    -> ON player (lastupdate) 
    -> ; 
Query OK, 0 rows affected (0.13 sec) 
Records: 0 Duplicates: 0 Warnings: 0 
mysql> explain 
    -> SELECT id,score,signupdate FROM player 
    -> JOIN matches ON matches.playerid = player.id 
    -> WHERE lastupdate = curdate() 
    -> ; 
+----+-------------+---------+------+---------------+---------------+---------+-----------------+------+-------------+ 
| id | select_type | table | type | possible_keys | key   | key_len | ref    | rows | Extra  | 
+----+-------------+---------+------+---------------+---------------+---------+-----------------+------+-------------+ 
| 1 | SIMPLE  | player | ref | player_idx_2 | player_idx_2 | 3  | const   | 1 |    | 
| 1 | SIMPLE  | matches | ref | matches_idx_1 | matches_idx_1 | 4  | mysql.player.id | 1 | Using index | 
+----+-------------+---------+------+---------------+---------------+---------+-----------------+------+-------------+ 
2 rows in set (0.00 sec) 

mysql> 

Answer 2

绝对数字2.主键用于唯一标识一行，并且id属性足够用于该属性，因此您不需要选项3.并且由于大多数查询看起来像您所说的，因此有lastupdate上的索引对于加快您的查询速度绝对有用。