先看一下arena_match_index的表結構，大家注意表的索引結構
CREATE TABLE `arena_match_index` (
`tid` int(10) unsigned NOT NULL DEFAULT '0',
`mid` int(10) unsigned NOT NULL DEFAULT '0',
`group` int(10) unsigned NOT NULL DEFAULT '0',
`round` tinyint(3) unsigned NOT NULL DEFAULT '0',
`day` date NOT NULL DEFAULT '0000-00-00',
`begintime` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
UNIQUE KEY `tm` (`tid`,`mid`),
KEY `mid` (`mid`),
KEY `begintime` (`begintime`),
KEY `dg` (`day`,`group`),
KEY `td` (`tid`,`day`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8
接著看下面的sql：
SELECT round FROM arena_match_index WHERE `day` = '2010-12-31' AND `group` = 18 AND `begintime` < '2010-12-31 12:14:28' order by begintime LIMIT 1;
這條sql的查詢條件顯示可能使用的索引有`begintime`和`dg`，但是由于使用了order by begintime排序mysql最后選擇使用`begintime`索引，explain的結果為：
mysql> explain SELECT round FROM arena_match_index WHERE `day` = '2010-12-31' AND `group` = 18 AND `begintime` < '2010-12-31 12:14:28' order by begintime LIMIT 1;
+----+-------------+-------------------+-------+---------------+-----------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------------+-------+---------------+-----------+---------+------+--------+-------------+
| 1 | SIMPLE | arena_match_index | range | begintime,dg |<STRONG> </STRONG>begintime<STRONG> </STRONG>| 8 | NULL | 226480 | Using where |
+----+-------------+-------------------+-------+---------------+-----------+---------+------+--------+-------------+
explain的結果顯示使用`begintime`索引要掃描22w條記錄，這樣的查詢性能是非常糟糕的，實際的執行情況也是初次執行（還未有緩存數據時）時需要30秒以上的時間。實際上這個查詢使用`dg`聯合索引的性能更好，因為同一天同一個小組內也就幾十場比賽，因此應該優先使用`dg`索引定位到匹配的數據集合再進行排序，那么如何告訴mysql使用指定索引呢？使用use index語句：
mysql> explain SELECT round FROM arena_match_index use index (dg) WHERE `day` = '2010-12-31' AND `group` = 18 AND `begintime` < '2010-12-31 12:14:28' order by begintime LIMIT 1;
+----+-------------+-------------------+------+---------------+------+---------+-------------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------------+------+---------------+------+---------+-------------+------+-----------------------------+
| 1 | SIMPLE | arena_match_index | ref | dg | dg | 7 | const,const | 757 | Using where; Using filesort |
+----+-------------+-------------------+------+---------------+------+---------+-------------+------+-----------------------------+
explain結果顯示使用`dg`聯合索引只需要掃描757條數據，性能直接提升了上百倍，實際的執行情況也是幾乎立即就返回了查詢結果。 在最初的查詢語句中只要把order by begintime去掉，mysql就會使用`dg`索引了，再次印證了order by會影響mysql的索引選擇策略！
mysql> explain SELECT round FROM arena_match_index WHERE `day` = '2010-12-31' AND `group` = 18 AND `begintime` < '2010-12-31 12:14:28' LIMIT 1;
+----+-------------+-------------------+------+---------------+------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------------+------+---------------+------+---------+-------------+------+-------------+
| 1 | SIMPLE | arena_match_index | ref | begintime,dg | dg | 7 | const,const | 717 | Using where |
+----+-------------+-------------------+------+---------------+------+---------+-------------+------+-------------+
通過上面的例子說mysql有時候也并不聰明，并非總能做出最優選擇，還是需要我們開發者對它進行“調教”！
