mysql什么时候使用联合索引_mysql何时使用组合索引

create table test(

id1 int ,

id2 int,

id3 int,

id4 int,

key index_id12(id1,id2)

);

用到索引

explain select * from test where id1 < 10;

用到索引

explain select * from test where id1 < 10 and id2 > 1;

用到索引

explain select * from test where id2 > 1 and id1 < 2;

用到索引

explain select id1 from test order by id1 desc ;

explain select id1,id2 from test order by id1 desc ;

未用到索引

explain select * from test order by id1 desc ;

未用到索引

explain select * from test where id2 > 1;

未用到索引

explain select * from test order by id1 desc ;

未用到索引

explain select id1,id2,id3 from test order by id1 desc ;

组合索引与order by的相互使用影响不是很熟悉，求大神留言解答