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I recently had to rewrite a query that was running very slowly. It was taking over 30 seconds.
Upon analyzing it, I saw that it had 2 correlated sub-queries in it, and each was doing a count(*).
This meant that for each row in the main query, the two inner queries were running. In this case, the table the outer query was running in had just been increased from about 300 records to about 1500, so suddenly the query was running slowly. This kind of problem becomes more apparent as the number of rows in the outer query increases.
Here is the original query:
select automobiles.name, automobiles.model , (select count(*) from manufacturers inner join locatables on manufacturers.id = locatables.locatables_id where automobiles.id = locatables.location_id and locatables.locatables_type = 'domestic' and manufacturers.deleted_at is null) as manufacturers_count, (select count(*) from locations inner join locatables on locations.id = locatables.locatables_id where automobiles.id = locatables.location_id and locatables.locatables_type = 'domestic' and locations.deleted_at is null) as locations_count from automobiles where automobiles.deleted_at is null
And here is the refactored version:
select manufacturer_count.cnt as manufacturer_count, location_count.cnt as location_count, a.name, a.model from automobiles a join( select l.location_id, count(l.locatables_id) as cnt from manufacturers m inner join locatables l on m.id = l.locatables_id and l.locatables_type = 'domestic' and m.deleted_at is null where location_id is not null group by l.location_id order by count(l.locatables_id) desc ) manufacturer_count on a.id = manufacturer_count.location_id join( select l.location_id, count(l.locatables_id) as cnt from locations c inner join locatables l on c.id = l.locatables_id and l.locatables_type = 'domestic' and c.deleted_at is null where location_id is not null group by l.location_id order by count(l.locatables_id) desc ) location_count on a.id = location_count.location_id
What I did was turn each of the sub-queries into a derived table, and then joined on those tables. To do this, I needed to have a column to match on in the join.
Because they were from my work, the queries I show here are similar, but not exactly the same as the real ones I used. I changed the table names and other values.
A great resource for learning about derived tables can be found at http://www.mysqltutorial.org/mysql-derived-table.