create table t1(a int primary key , b int , c int, index(b,c));
insert into t1 values(1,1,1),(2,2,2),(3,3,3),(4,4,4),(5,5,5);
insert into t1 select a+5, b+5, c+5 from t1;
insert into t1 select a+10, b+10, c+10 from t1;
insert into t1 select a+20, b+20, c+20 from t1;
insert into t1 select a+40, b+40, c+40 from t1;
explain select * from t1 where a=34;;
id	1
select_type	SIMPLE
table	t1
type	const
possible_keys	PRIMARY
key	PRIMARY
key_len	4
ref	const
rows	1
Extra	
explain select * from t1 where b=34 and c = 34;;
id	1
select_type	SIMPLE
table	t1
type	ref
possible_keys	b
key	b
key_len	10
ref	const,const
rows	1
Extra	
explain select * from t1 where c=34;;
id	1
select_type	SIMPLE
table	t1
type	ALL
possible_keys	NULL
key	NULL
key_len	NULL
ref	NULL
rows	80
Extra	Using where
drop table t1;