mysql选择索引 1、尽量为用来搜索、分类或分组的数据列编制索引，不要为作为输出显示的数据列编制索引。 最适合有索引的数据列是那些在where子句中数据列，在联结子句中出现的数据列，或者是在Group by 、Order by子句中出现的数据列。select 后的数据列最好
mysql选择索引1、尽量为用来搜索、分类或分组的数据列编制索引，不要为作为输出显示的数据列编制索引。
最适合有索引的数据列是那些在where子句中数据列，在联结子句中出现的数据列，或者是在Group by 、Order by子句中出现的数据列。select 后的数据列最好不要用索引。
2、综合考虑各数据列的维度。
数据列的维度等于它所容纳的非重复值的个数。比如说，如果某个数据列的值是1,2,3,3,4。那么它的维度就是3.维度越高，越适合做索引。原因不言而喻。
3、对短小的值进行索引。
4、为字符串的前缀编索引。
5、充分利用最左边的前缀。
当你创建了一个n个数据列的符合索引时，因为索引中最左边的数据列集合能够使用的n个索引。一个复合索引在工作时就相当于n个索引，因为索引最左边的数据列集合能够用于匹配数据行。这样的一个集合就相当于“最左边前缀”。MYSQL不能使用没有包含最左边前缀的搜索的索引。
6、适可而止，不要建立过多的索引。
索引会占用额外的磁盘空间，会影响写的性能。当你索引过多时，mysql甚至无法选择最好的索引来使用。
7、让索引的类型与你打算进行的比较操作的类型保持匹配。(使用B树索引的关键字:USING BTREE).
8、利用慢查询日志找出性能低劣的查询。（慢查询日志文件是一个文本文件。）
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