Performance at finding all foreigns with 3 one-to-many relation

Question

Basically, I have 2 tables : user and object. One user can be associated to one object by three ways (roles). I want to list all users associated at least to one business object by any role.

This query works, but it takes 5 seconds !

SELECT u.id
FROM user u
LEFT JOIN translation_revision t1
    ON u.id = t1.translator_id
LEFT JOIN translation_revision t2
    ON u.id = t2.proofreader_id
LEFT JOIN translation_revision t3
    ON u.id = t3.reviewer_id
wHERE t1.id IS NOT NULL
    OR t2.id IS NOT NULL
    OR t3.id IS NOT NULL
GROUP BY u.id   

Complete fiddle with creation and data here : https://www.db-fiddle.com/f/nojYaUsWicKJDuRQk1Nqd2/7

Any idea on how to improve perf here ?

Answer 1

Mine is fast, at 1-2 milliseconds (tested across various versions between 5.5 and 8.0, plus MariaDB):

SELECT u.id
FROM user u
WHERE EXISTS ( SELECT 1 FROM translation_revision t1
        WHERE u.id = t1.translator_id )
   OR EXISTS ( SELECT 1 FROM translation_revision t2
        WHERE u.id = t2.proofreader_id )
   OR EXISTS ( SELECT 1 FROM translation_revision t3
        WHERE u.id = t3.reviewer_id )
GROUP BY u.id   
LIMIT 30;

(I would prefer to use DISTINCT instead of GROUP BY, but probably no performance diff.)

Answer 2

Instead of doing 3 separate joins, you could accomplish the same thing by simply using an OR clause (the execution time seems to be ~1-2ms):

SELECT DISTINCT u.id
FROM user u
JOIN translation_revision t1
    ON u.id = t1.translator_id OR u.id = t1.proofreader_id OR u.id = t1.reviewer_id
GROUP BY u.id   
LIMIT 30

Other ideas would be to denormalize your data and keep some additional info about your users roles.

Answer 3

Maybe simply

SELECT DISTINCT u.id
FROM user u
JOIN translation_revision t 
    ON u.id IN (t.translator_id, t.proofreader_id, t.reviewer_id)

?

Answer 4

You may use union all to avoid sorting

mysql> select id from (
->               SELECT u.id
->               FROM user u
->               LEFT JOIN translation_revision t1
->               ON u.id = t1.translator_id  
->union all
->          SELECT u.id
->          FROM user u
->          LEFT JOIN translation_revision t2
->          ON u.id = t2.proofreader_id 
->union all
->         SELECT u.id
->         FROM user u
->         LEFT JOIN translation_revision t3
->         ON u.id = t3.reviewer_id 
-> ) t
->   group by id
-> ;
+----+
| id |
+----+
|  1 |
|  2 |
|  3 |
+----+
3 rows in set (0.01 sec)