SQL慢查询优化

表数据量比较大，sql很慢，该加的索引都加了，还是慢，偶然改变了下字段顺序，发现速度快了好多倍，看来where条件字段顺序真的很重要，按照范围大小，数据量大的放在前面，一层层过滤缩小范围是最优方式


select uid, path, down, shareTime from grant where type = 'G' and sid in (34750,36297,37504,37505,37506,37507,37508,37509,37510,37511,37512,37513,41258) and share = 1 and isdir = '0' order by createdTime desc limit 100 offset 0;
修改为：
select uid, path, down, shareTime from grant
where sid in (34750,36297,37504,37505,37506,37507,37508,37509,37510,37511,37512,37513,41258) and type = 'G' and isdir = 0 and share = 1 
order by createdTime desc 
limit 100 offset 0
查询速度快近70倍。


同时还要注意一个地方，就是order by createdTime desc 
因为创建时间和id主键是关联的，这里大可不必用创建时间，直接用id desc
这个更不得了，基本上就毫无压力了