Mysql long execution query - mysql

Question

I have table with 38k rows and I use this query to compare item id from items table with item id from posted_domains table.
select * from `items`
where `items`.`source_id` = 2 and `items`.`source_id` is not null
and not exists (select *
from `posted_domains`
where `posted_domains`.`item_id` = `items`.`id` and `domain_id` = 1)
order by `item_created_at` asc limit 1
This query took 8s. I don't know if is a problem with my query or my mysql is bad configured. This query is generated by Laravel relations like
$items->doesntHave('posted', 'and', function ($q) use ($domain) {
$q->where('domain_id', $domain->id);
});

Answer 1

CORRELATED subqueries can be rather slow (as they are often executed repeatedly, once for each row in the outer query), this might be faster.
select *
from `items`
where `items`.`source_id` = 2
and `items`.`source_id` is not null
and item_id not in (
select DISTINCT item_id
from `posted_domains`
where `domain_id` = 1)
order by `item_created_at` asc
limit 1
I say might because subqueries in where are also rather slow in MySQL.
This LEFT JOIN will probably be the fastest.
select *
from `items`
LEFT JOIN (
select DISTINCT item_id
from `posted_domains`
where `domain_id` = 1) AS subQ
ON items.item_id = subQ.item_id
where `items`.`source_id` = 2
and `items`.`source_id` is not null
and subQ.item_id is null
order by `item_created_at` asc
limit 1;
Since it is a no matches scenario, it technically doesn't even need to be a subquery; and might be faster as a direct left join, but that will depend on indexes, and possibly actual data values.