?轉自：http://www.cnblogs.com/edwardlost/archive/2010/12/31/1923105.html

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? | begintime | 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”！

轉載于:https://blog.51cto.com/leloup/610056

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