1.5个字段建了联合索引的，他们的规则是怎么样的。什么情况会用上什么情况用不上
2.如果两个字段是单独索引，where a=x order b大家觉得用到哪个索引了， 还where a=x order by C 这样用到索引了，C字段是没有索引的。
现在不太方便测试。先谢过大神
回复内容：
1.5个字段建了联合索引的，他们的规则是怎么样的。什么情况会用上什么情况用不上
2.如果两个字段是单独索引，where a=x order b大家觉得用到哪个索引了， 还where a=x order by C 这样用到索引了，C字段是没有索引的。
现在不太方便测试。先谢过大神
命名规则：表名_字段名
1、需要加索引的字段，要在where条件中
2、数据量少的字段不需要加索引
3、如果where条件中是OR关系，加索引不起作用
4、符合最左原则
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