MySQL 8.0

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create table test (
id int not null auto_increment primary key,
dt datetime,
hdt int as (hour(dt)),
index idx1 (dt),
index idx2 (hdt)
)
insert into test (dt)
with recursive
cte as (select 1 num
        union all
        select num+1 from cte where num < 1000)
select now() + interval num minute from cte
explain 
select *
from test
where hour(dt)=0


id
select_type
table
partitions
type
possible_keys
key
key_len
ref
rows
filtered
Extra


1
SIMPLE
test
null
ref
idx2
idx2
5
const
60
100.00
null


explain 
select *
from test
where hdt=0


id
select_type
table
partitions
type
possible_keys
key
key_len
ref
rows
filtered
Extra


1
SIMPLE
test
null
ref
idx2
idx2
5
const
60
100.00
null