Extremely slow query - mysql

Question

Below is a query which takes 30+ seconds to run. Based on similar queries I have running, I can't see where the hold up is here. My only thought is joining the job user id to job_applicants user id, but they need to be mapped.
SELECT DISTINCT u.user_id, u.first_name, u.last_name FROM users u
LEFT OUTER JOIN employee_access ea ON ea.user_id = u.user_id
LEFT OUTER JOIN confirmation c ON c.user_id = u.user_id
LEFT OUTER JOIN job_applicants a ON a.user_id = u.user_id
LEFT OUTER JOIN job j ON j.job_id = a.job_id
WHERE ea.access_id = 4 OR c.access_id = 4 OR (a.process_level = 0 AND j.access_id = 4)
ORDER BY u.last_name asc

Answer 1

Use exists:
select u.*
from users u
where exists (select 1
from employee_access ea
where ea.user_id = u.user_id and ea.access_id = 4
) or
exists (select 1
from confirmation c
where c.user_id = u.user_id and c.access_id = 4
) or
exists (select 1
from job_applicants a join
job j
on j.job_id = a.job_id
where a.user_id = u.user_id and
a.process_level = 0 AND j.access_id = 4
)
order by u.last_name;
This will prevent all the Cartesian products and the final removal of duplicates.
I would recommend indexes on:
users(last_name, user_id)
employee_access(user_id, access_id)
confirmation(user_id, access_id)
job_applicants(user_id, process_level, job_id)
job(job_id, access_id)

Answer 2

Yet another approach. This has the advantage of first gathering the list of user_ids, then reaching into users for the other columns:
SELECT u.user_id, u.first_name, u.last_name
FROM users u
JOIN (
( SELECT user_id FROM employee_access WHERE access_id = 4 )
UNION DISTINCT
( SELECT user_id FROM confirmation WHERE access_id = 4 )
UNION DISTINCT
( SELECT a.user_id
FROM job_applicants a
JOIN job j USING(job_id)
WHERE a.process_level = 0
AND j.access_id = 4 )
) AS x USING(user_id)
ORDER BY u.last_name ASC
Indexes:
employee_access: INDEX(access_id, user_id) -- (covering)
confirmation: INDEX(access_id, user_id) -- (covering)
job: INDEX(access_id, job_id) -- (covering)
job_applicants: INDEX(process_level, job_id, user_id) -- (covering)
users: PRIMARY KEY(user_id)
See if this will shave off most of the remaining 8 seconds.

Answer 3

This should work. It is similar in concept to Gordon's answer but I have a borderline pathological distrust of correlated subqueries.
SELECT DISTINCT u.user_id, u.first_name, u.last_name
FROM users u
WHERE u.user_id IN (SELECT user_id FROM employee_access WHERE access_id = 4)
OR u.user_id IN (SELECT user_id FROM confirmation WHERE access_id = 4)
OR u.user_id IN (
SELECT a.user_id
FROM job_applicants a
INNER JOIN job j ON j.job_id = a.job_id
WHERE a.process_level = 0 AND j.access_id = 4
)
ORDER BY u.last_name asc