MySQL 5.5

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select version();


version()


5.5.62


create database d;
use d;
create table t (id int auto_increment primary key, a int, b int);
create index ab on t (a ASC, b ASC);

Records: 0  Duplicates: 0  Warnings: 0


explain select a, b from t where a = 1 and b = 1;


id
select_type
table
type
possible_keys
key
key_len
ref
rows
Extra


1
SIMPLE
t
ref
ab
ab
10
const,const
1
Using where; Using index


explain select a, b from t where b = 1 and a = 1;


id
select_type
table
type
possible_keys
key
key_len
ref
rows
Extra


1
SIMPLE
t
ref
ab
ab
10
const,const
1
Using where; Using index