Use index to optimize SQL query "suggestions collection"

Hello everyone , I meet you again , I'm the king of the whole stack .

Have a look first arena_match_index The table structure , Pay attention to the index structure of the table

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

Copy

Then look at the following 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;

Copy

This article sql The query condition of shows that the possible indexes used are `begintime` and `dg`, But because of use order by begintime Sort mysql Last choice use `begintime` Indexes ,explain As the result of the ：

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 |
+----+-------------+-------------------+-------+---------------+-----------+---------+------+--------+-------------+

Copy

explain The results of display use `begintime` Index to scan 22w Bar record , Such query performance is very poor , The actual implementation is also the first implementation （ When there is no cached data ） The need when 30 Seconds or more . In fact, this query uses `dg` The performance of Federated index is better , Because there are only dozens of games in the same group on the same day , Therefore, priority should be given to `dg` The index locates the matching data set and then sorts , So how to tell mysql Use the specified index ？ Use use index sentence ：

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 |
+----+-------------+-------------------+------+---------------+------+---------+-------------+------+-----------------------------+

Copy

explain The results show that using `dg` The federated index only needs to be scanned 757 Data , The performance has been directly improved hundreds of times , The actual execution also returns the query results almost immediately .

In the initial query statement, just put order by begintime Get rid of ,mysql Will use `dg` Index , It is confirmed again order by It will affect mysql Index selection strategy ！

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 |
+----+-------------+-------------------+------+---------------+------+---------+-------------+------+-------------+

Copy

Say through the above example mysql Sometimes it's not smart , Not always make the best choice , We still need developers to do it “ Training ”！

Publisher ： Full stack programmer stack length , Reprint please indicate the source ：https://javaforall.cn/111652.html Link to the original text ：https://javaforall.cn