How can this SQL query be optimized? (Running on MySQL) - mysql

Question

I need help on rewriting an SQL query that takes 26 seconds to run on an MySQL server.
The query is:
select
c.countries_name,
c.country_id,
(SELECT
count(1)
FROM
`fav_country`
WHERE
`country_id`=c.country_id
and device_id='".$device_id."'
) as isFav,
c.image,
c.countries_iso_code,
s.country
from
station s
left join countries c on c.country_id=s.country
where
isactive=:isactive
group by
s.country
I have tried rewriting it with two left/right joins but to no avail.
Basically, we have three tables, countries, fav_country and station, the common field is the country id (countries.country_id, fav_country.countr_id and station.country)
Thanks in advance!

Answer 1

This is the query you want optimized:
select c.countries_name, c.country_id,
(select count(1)
from fav_country fc
where fc.country_id = c.country_id and
fc.device_id = ? -- use a parameter!
) as isFav,
c.image, c.countries_iso_code
from station s left join
countries c
on c.country_id = s.country
where s.isactive = :isactive
group by s.country;
First, avoiding the outer aggregate is very helpful. I would replace it with exists:
select c.countries_name, c.country_id,
(select count(1)
from fav_country fc
where fc.country_id = c.country_id and
fc.device_id = ?
) as isFav,
c.image, c.countries_iso_code
from countries c
where exists (select 1
from station s
where s.country = c.country_id and
s.isactive = :isactive
);
Then for this query, you want indexes on:
station(country, isactive)
fav_country(country_id, device_id).