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