Very slow MySQL query (Left-JOIN, GROUP BY, etc. ?) - mysql

Question

I have troubles with the subsequent query, which I submit to a MySQL server. It takes 25s... for (COUNT(*) < 20k)-tables - only featured have 600k rows. However, indexes are created where it should (that is to say : for concerned columns in ON clauses). I tried to remove GROUP BY, which improved the case a bit. But the queries still give a slow response a general rule. I made that post because I could not find a solution into the variety of cases found into stackoverflow. Any suggestion?
SELECT
doctor.id as doctor_id,
doctor.uuid as doctor_uuid,
doctor.firstname as doctor_firstname,
doctor.lastname as doctor_lastname,
doctor.cloudRdvMask as doctor_cloudRdvMask,
GROUP_CONCAT(recommendation.id SEPARATOR ' ') as recommendation_ids,
GROUP_CONCAT(recommendation.uuid SEPARATOR ' ') as recommendation_uuids,
GROUP_CONCAT(recommendation.disponibility SEPARATOR ' ') as recommendation_disponibilities,
GROUP_CONCAT(recommendation.user_id SEPARATOR ' ') as recommendation_user_ids,
GROUP_CONCAT(recommendation.user_uuid SEPARATOR ' ') as recommendation_user_uuids,
location.id as location_id,
location.uuid as location_uuid,
location.lat as location_lat,
location.lng as location_lng,
profession.id as profession_id,
profession.uuid as profession_uuid,
profession.name as profession_name
FROM featured as doctor
LEFT JOIN location as location
ON doctor.location_id = location.id
LEFT JOIN profession as profession
ON doctor.profession_id = profession.id
LEFT JOIN
(
SELECT
featured.id as id,
featured.uuid as uuid,
featured.doctor_id as doctor_id,
featured.disponibility as disponibility,
user.id as user_id,
user.uuid as user_uuid
FROM featured as featured
LEFT JOIN user as user
ON featured.user_id = user.id
WHERE discr = 'recommendation'
) as recommendation
ON recommendation.doctor_id = doctor.id
WHERE
doctor.discr = 'doctor'
AND
doctor.state = 'Publié'
GROUP BY doctor.uuid
Here comes the EXPLAIN result:
id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
1 | SIMPLE | doctor | NULL | ref | discr,state | discr | 767 | const | 194653 | 50.00 | Using where |
1 | SIMPLE | location | NULL | eq_ref | PRIMARY | PRIMARY | 4 | doctoome.doctor.location_id | 1 | 100.00 | NULL |
1 | SIMPLE | profession | NULL | eq_ref | PRIMARY | PRIMARY | 4 | doctoome.doctor.profession_id | 1 | 100.00 | NULL |
1 | SIMPLE | featured | NULL | ref | IDX_3C1359D487F4FB17,discr | IDX_3C1359D487F4FB17 | 5 | doctoome.doctor.id | 196 | 100.00 | Using where |
1 | SIMPLE | user | NULL | eq_ref | PRIMARY | PRIMARY | 4 | doctoome.featured.user_id | 1 | 100.00 | Using index |
EDIT This link helped me, it goes now with 8s. https://www.percona.com/blog/2016/10/12/mysql-5-7-performance-tuning-immediately-after-installation/. But I still find it slow, I just let it in case anybody would know what could also be improved. Thanks

Answer 1

I think removing the subquery might help, along with some more indexes:
SELECT . . . -- you need to fix the `GROUP_CONCAT()` column references
FROM featured doctor LEFT JOIN
location location
ON doctor.location_id = location.id LEFT JOIN
profession profession
ON doctor.profession_id = profession.id LEFT JOIN
featured featured
ON featured.doctor_id = doctor.doctor_id LEFT JOIN
user user
ON featured.user_id = user.id
WHERE doctor.discr = 'doctor' AND
doctor.state = 'Publié' AND
featured.discr = 'recommendation'
GROUP BY doctor.uuid;
Then you want an index on featured(discr, state, doctor_id, location_id, profession_id) and featured(doctor_id, discr, user_id).