Optimize COUNT(*) - mysql

Question

I have a table items from which I'm selecting 40 rows at a time ordered by the popularity of the item.
The popularity score is simply downloads/impressions;
Query:
SELECT id, name
FROM items
ORDER BY (SELECT COUNT(*) FROM downloads WHERE item = items.id)/
(SELECT COUNT(*) FROM impressions WHERE item = items.id)
LIMIT 40;
The problem is that the query takes forever to complete (ranging from 2 to 10 seconds).
At the moment we have 25K items, 18M impressions, and 560k download.
We already tried adding the fields downloads and impressions in the table items and keeping the count updated using triggers (after an insert in the tables impressions and downloads we increment the values), but we've had some issues with deadlocking.
Is there a better way to optimize this query?
Thanks.
Edit
Here's the output of EXPLAIN
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY items ALL NULL NULL NULL NULL 20496 Using filesort
3 DEPENDENT SUBQUERY impressions ref PRIMARY PRIMARY 4 db.items.id 74 Using index
2 DEPENDENT SUBQUERY downloads ref PRIMARY PRIMARY 4 db.items.id 274 Using index
Tables:
CREATE TABLE `items` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(35) DEFAULT '',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=24369 DEFAULT CHARSET=utf8mb4;
CREATE TABLE `impressions` (
`item` int(10) unsigned NOT NULL,
`user` char(36) NOT NULL DEFAULT '',
PRIMARY KEY (`item`,`user`),
CONSTRAINT `impression_ibfk_1` FOREIGN KEY (`item`) REFERENCES `items` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
CREATE TABLE `downloads` (
`item` int(10) unsigned NOT NULL,
`user` char(36) NOT NULL DEFAULT '',
PRIMARY KEY (`item`,`user`),
CONSTRAINT `download_ibfk_1` FOREIGN KEY (`item`) REFERENCES `items` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

Answer 1

I think next query can resolve your problem:
SELECT
item,items.name, downloads.cnt/impressions.cnt AS rate
FROM (
SELECT item, COUNT(*) AS cnt FROM downloads GROUP BY item
) AS downloads
JOIN (
SELECT item, COUNT(*) AS cnt FROM impressions GROUP BY item
) impressions
JOIN items ON items.id = downloads.items
ORDER BY rate DESC
LIMIT 40;
Also care about downloads and impressions tables have indexed by item field.

Answer 2

Not solvable with that approach.
There are two solutions:
Keep counters (by item.id) for impressions and downloads.
Summary tables.
Counters This involves adding an extra column for each counter to the items table. Or building a parallel table with id and the various counters. For really high volumn of counts, the latter avoids some clashes between various queries.
Summary Tables Build and incrementally augment a table(s) that summarize counts like these, plus perhaps other SUMs, COUNTs, etc. The table would perhaps be augmented daily for the previous day's information. Then the "sum the counts" to get the grand total; this will be much faster than your current query.
More on Summary Tables: http://mysql.rjweb.org/doc.php/summarytables

Answer 3

I'd count downloads and impressions first and then get the top 40:
with d as (select item, count(*) as total from downloads group by item)
, i as (select item, count(*) as total from impressions group by item)
, top40 as select item from d join i using (item) order by d.total / i.total limit 40)
select *
from items
where id in
(
select item from top40
);
The WITH clause is available as of MySQL 8. In earlier versions, you'd work with subqueries instead.
As item is a foreign key in downloads and impressions and id is the primary key in items, I suppose there is an index on them. Otherwise create one:
create unique index idx1 on items(id);
create index idx2 on downloads(item);
create index idx3 on impressions(item);