Speed up SQL Query with a multiple inner joins

Question

I have the following query :

SELECT COUNT(DISTINCT(`person_id`)) as `count`, `mc_office`.`name` as `office_name`
FROM `aft_people` 
                      INNER JOIN `aft_offices` 
                      ON `aft_people`.`lc`=`aft_offices`.`id`
                      INNER JOIN `aft_offices` as `mc_office`
                      ON `aft_offices`.`parent_id` = `mc_office`.`id`
                      INNER JOIN `aft_constant_maps` as `constant_maps`
                      ON `aft_people`.`person_id` = `constant_maps`.`representable_id`
                      WHERE `constant_maps`.`constant_id` IN (741)
                      GROUP BY `mc_office`.`name`

Table aft_people has 1M records and aft_constant_maps has around 5M records. There are indexes on the fields

• aft_people.person_id
• aft_constant_maps.representable_id
• aft_constant_maps.constant_id

The query is really really slow and sometimes it doesn't even load at all. I need this query to execute in less than 10 seconds.

Do let me know if you would like more information to help me out.

Answer 1

For this query (I've just rearranged the join's so I can follow them better):

SELECT COUNT(DISTINCT(`person_id`)) as `count`,
       `mc_office`.`name` as `office_name`
from `aft_people` INNER JOIN 
     `aft_constant_maps` as `constant_maps`
     ON `aft_people`.`person_id` = `constant_maps`.`representable_id` INNER JOIN
     `aft_offices` 
     ON `aft_people`.`lc` = `aft_offices`.`id` INNER JOIN
     `aft_offices` as `mc_office`
     ON `aft_offices`.`parent_id` = `mc_office`.`id` 
WHERE `constant_maps`.`constant_id` IN (741)
GROUP BY `mc_office`.`name`

The best indexes are: constant_maps(constant_id, representable_id) , aft_people(person_id, lc) , and aft_offices(id, parent_id) . These might help speed up the query.