mysql查询起先慢后快

Question

我有2个myISAM表，分别称为“测试”和“ completed_tests”，一个表有170个条目，另一个表有118k条目。当我运行此查询时：

SELECT ct.archive, ct.status, ct.score, ct.users_LOGIN, t.lessons_ID, t.content_ID, t.keep_best 
 FROM completed_tests ct,tests t 
WHERE ct.status != 'deleted' and ct.status != 'incomplete' and t.id=ct.tests_ID and t.lessons_ID=10;

然后大约需要30秒才能完成。随后对同一查询或相关查询的调用（例如，不同的lessons_ID）更快。即使我重置查询缓存或重新启动mysql服务器，它们仍然保持更快的速度。我想这意味着表被缓存到内存中（并留在那里）。我的问题是，该特定查询似乎在运行此应用程序的高流量站点上引起了问题（我想这是因为服务器的内存不足并清空了其缓存？）。我的问题是：

• 有没有办法在我的系统上一致地复制30英寸的延迟，所以我可以尝试优化查询？例如，是否应该清空系统的缓存？
• 有没有一种方法可以优化上面的查询？运行解释会给出：

运行说明给出：

mysql> explain SELECT ct.archive, ct.status, ct.score, ct.users_LOGIN, t.lessons_ID, t.content_ID, t.keep_best FROM completed_tests ct,tests t WHERE ct.status != 'deleted' and ct.status != 'incomplete' and t.id=ct.tests_ID and t.lessons_ID=10;
+----+-------------+-------+------+-----------------+----------+---------+---------------+------+-------------+
| id | select_type | table | type | possible_keys   | key      | key_len | ref           | rows | Extra       |
+----+-------------+-------+------+-----------------+----------+---------+---------------+------+-------------+
|  1 | SIMPLE      | t     | ref  | PRIMARY,idx1    | idx1     | 3       | const         |    4 |             |
|  1 | SIMPLE      | ct    | ref  | tests_ID,status | tests_ID | 3       | firstcho.t.id | 1025 | Using where |
+----+-------------+-------+------+-----------------+----------+---------+---------------+------+-------------+

据我了解，这表明成功使用了索引。谢谢大家。

表结构

>show create table 'tests';
CREATE TABLE `tests` (
  `id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
  `active` tinyint(1) NOT NULL DEFAULT '1',
  `content_ID` mediumint(8) unsigned NOT NULL DEFAULT '0',
  `lessons_ID` mediumint(8) unsigned NOT NULL DEFAULT '0',
  `name` varchar(255) NOT NULL DEFAULT '',
  `mastery_score` tinyint(4) unsigned NOT NULL DEFAULT '0',
  `description` text,
  `options` text,
  `publish` tinyint(1) DEFAULT '1',
  `keep_best` tinyint(1) DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `idx1` (`lessons_ID`)
) ENGINE=MyISAM AUTO_INCREMENT=171 DEFAULT CHARSET=utf8

>show create table completed_tests;
CREATE TABLE `completed_tests` (
  `id` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
  `users_LOGIN` varchar(100) DEFAULT NULL,
  `tests_ID` mediumint(8) unsigned NOT NULL DEFAULT '0',
  `test` longblob,
  `status` varchar(255) DEFAULT NULL,
  `timestamp` int(10) unsigned NOT NULL DEFAULT '0',
  `archive` tinyint(1) NOT NULL DEFAULT '0',
  `time_start` int(10) unsigned DEFAULT NULL,
  `time_end` int(10) unsigned DEFAULT NULL,
  `time_spent` int(10) unsigned DEFAULT NULL,
  `score` float DEFAULT NULL,
  `pending` tinyint(1) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `users_login` (`users_LOGIN`),
  KEY `tests_ID` (`tests_ID`),
  KEY `status` (`status`),
  KEY `timestamp` (`timestamp`),
  KEY `archive` (`archive`),
  KEY `score` (`score`),
  KEY `pending` (`pending`)
) ENGINE=MyISAM AUTO_INCREMENT=117996 DEFAULT CHARSET=utf8

Answer 1

最后，我将表拆分为 2 个，将 blob 对象移动到第二个表，并在需要的地方加入。不幸的是，这涉及到更改许多行代码。