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MySQL手册版本 5.0.20-MySQL优化(二) (1)(3)

Jun 13, 2016 am 10:06 AM
mysql Same different two optimization manual scanning Version of type index connect



连接类型跟 ALL 一样,不同的是它只扫描索引树。它通常会比 ALL 快点,因为索引文件通常比数据文件小。MySQL在查询的字段知识单独的索引的一部分的情况下使用这种连接类型。


ALL


将对该表做全部扫描以和从前一个表中取得的记录作联合。这时候如果第一个表没有被标识为 const 的话就不大好了,在其他情况下通常是非常糟糕的。正常地,可以通过增加索引使得能从表中更快的取得记录以避免 ALL。


possible_keys


possible_keys 字段是指MySQL在搜索表记录时可能使用哪个索引。注意,这个字段完全独立于 EXPLAIN 显示的表顺序。这就意味着 possible_keys 里面所包含的索引可能在实际的使用中没用到。如果这个字段的值是 NULL,就表示没有索引被用到。这种情况下,就可以检查 WHERE 子句中哪些字段那些字段适合增加索引以提高查询的性能。就这样,创建一下索引,然后再用 EXPLAIN 检查一下。详细的查看章节"14.2.2 ALTER TABLE Syntax"。想看表都有什么索引,可以通过 SHOW INDEX FROM tbl_name 来看。


key


key 字段显示了MySQL实际上要用的索引。当没有任何索引被用到的时候,这个字段的值就是 NULL。想要让MySQL强行使用或者忽略在 possible_keys 字段中的索引列表,可以在查询语句中使用关键字FORCE INDEX, USE INDEX, 或 IGNORE INDEX。如果是 MyISAM 和 BDB 类型表,可以使用 ANALYZE TABLE 来帮助分析使用使用哪个索引更好。如果是 MyISAM 类型表,运行命令 myisamchk --analyze 也是一样的效果。详细的可以查看章节"14.5.2.1 ANALYZE TABLE Syntax"和"5.7.2 Table Maintenance and Crash Recovery"。


key_len


key_len 字段显示了MySQL使用索引的长度。当 key 字段的值为 NULL 时,索引的长度就是 NULL。注意,key_len 的值可以告诉你在联合索引中MySQL会真正使用了哪些索引。

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