”排好序的快速查找结构”.
like , > ,< 范围查询,只是使用了索引的一半,联合索引的后半不会在使用
where order 里面从左往右查看是否索引有断的 index(c1,c2,c3,c4)
where c1=x and c2=x and c4>x and c3=x 都用
where c1=x and c2=x and c4=x order by c3; 这个用了c1,c2,c3,c4 所有索引
where c1=x and c4=x group by c3,c2
where c1=x and c5=x order by c2,c3
where c1=x and c2=x and c5=x order by c2,c3
