MySQL 8.0

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CREATE TABLE `datesort` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `value` int(11) NOT NULL,
  `created_at` datetime NOT NULL,
  PRIMARY KEY (`id`),
  KEY `c_v` (`created_at`,`value`)
) ENGINE=InnoDB;
EXPLAIN SELECT COUNT(*), `value`, created_at
FROM datesort
WHERE created_at > NOW() - INTERVAL 1 DAY
GROUP BY created_at, value;


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


1
SIMPLE
datesort
null
index
c_v
c_v
9
null
1
100.00
Using where; Using index


EXPLAIN SELECT COUNT(*), `value`,  date(created_at)
FROM datesort
WHERE created_at > NOW() - INTERVAL 1 DAY
GROUP BY date(created_at), `value`;


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


1
SIMPLE
datesort
null
index
c_v
c_v
9
null
1
100.00
Using where; Using index; Using temporary


alter table datesort
    add column date_created_at date
    generated always as (date(created_at)) stored
;

Records: 0  Duplicates: 0  Warnings: 0


create index myidx on datesort(date_created_at, value);

Records: 0  Duplicates: 0  Warnings: 0


explain 
select count(*) cnt, value,  date_created_at
from datesort
where date_created_at > current_date - interval 1 day
group by date_created_at, value;


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


1
SIMPLE
datesort
null
index
myidx
myidx
8
null
1
100.00
Using where; Using index