select * from t where city in ('杭州',"苏州") order by name limit 100;
假如已经设置了(city,name)的联合索引,这个查询还需要排序嘛?
select * from t where city in ('杭州',"苏州") order by name limit 100;
假如已经设置了(city,name)的联合索引,这个查询还需要排序嘛?
(city,name)联合索引包含city和city,name,上述SQL中where使用in查找无法名字city索引,所以order by name不会走联合索引,改成union查询命中联合索引正序排序直接是联合索引的顺序