Improve performance of a last status retrieval from history table - mysql

Question

I want to retrieve the latest status for an item from a history table. History table will have a record of all status changes for an item. The query must be quick to run.
Below is the query that I use to get the latest status per item
SELECT item_history.*
FROM item_history
INNER JOIN (
SELECT MAX(created_at) as created_at, item_id
FROM item_history
GROUP BY item_id
) as latest_status
on latest_status.item_id = item_history.item_id
and latest_status.created_at = item_history.created_at
WHERE item_history.status_id = 1
and item_history.created_at BETWEEN "2020-12-16" AND "2020-12-23"
I've tried putting query above into another inner join to link data with an item:
SELECT *
FROM `items`
INNER JOIN ( [query from above] )
WHERE items.category_id = 3
Notes about item_history table, I have index on the following columns: status_id, creatd_at and listing_id. I have also turned 3 of those into a compound primary key.
My issue is that MySQL keeps scanning the full table to grab MAX(created_at) which is a very slow operation, even tho I only have 3 million records within the history table.
Query plan as follows:
id
select_type
table
partitions
type
possible_keys
key
key_len
ref
rows
filtered
Extra
1
PRIMARY
items
NULL
ref
"PRIMARY,district"
district
18
const
694
100.00
NULL
1
PRIMARY
item_history
NULL
ref
"PRIMARY,status_id,created_at,item_history_item_id_index"
PRIMARY
9
"main.items.id,const"
1
100.00
"Using where"
1
PRIMARY
NULL
ref
<auto_key0>
<auto_key0>
14
"func,main.items.id"
10
100.00
"Using where; Using index"
2
DERIVED
item_history
NULL
range
"PRIMARY,status_id,created_at,item_history_item_id_index"
item_history_item_id_index
8
NULL
2751323
100.00
"Using index"

Answer 1

I want to retrieve the latest status for an item from a history table.
If you want the results for just one item, then use order by and limit:
select *
from item_history
where item_id = ? and created_at between '2020-12-16' and '2020-12-23'
order by created_at desc limit 1
This query would benefit an index on (item_id, created_at).
If you want the latest status per item, I would recommend a correlated subquery:
select *
from item_history h
where created_at = (
select max(h1.created_at)
from item_history h1
where h1.item_id = h.item_id
and h1.created_at between '2020-12-16' and '2020-12-23'
)
The same index should be beneficial.

Answer 2

Using window function MySQL 8.0.14+:
WITH cte AS (
SELECT *, ROW_NUMBER() OVER(PARTITION BY item_id ORDER BY created_at DESC) r
FROM item_history
WHERE item_history.status_id = 1
and item_history.created_at BETWEEN '2020-12-16' AND '2020-12-23'
)
SELECT *
FROM cte WHERE r = 1;
Index on (item_id,created_at) will also help