Question

I have two tables: users and user_friends with a 1:1 reflective join on the users table. (i.e. user_friends records are like [user_id, friend_id], [friend_id, user_id]).

Let us say I have a user called “self”. What I am trying to do is find all users who have a mutual friend in common with “self” and who meet some other simple criteria in the users table. This is the best I came up with so far, which runs in about 18 seconds.

Can this query be improved?

SELECT 
  DISTINCT(friend_id) 
FROM 
  user_friends, users 
WHERE 
  users.id != #{self.id}
AND (
  signed_up IS NOT NULL 
  OR 
  user_id IN (
    SELECT 
      friend_id 
    FROM 
      user_friends 
    WHERE 
      user_id = #{self.id}
  )
) 
AND 
  users.id = friend_id
AND 
  relationship_status = 'Single'
AND 
  current_location_id IS NOT NULL
;

In addition, here is the EXPLAIN PLAN for this query. Is the “temporary” table causing the problem?

*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: user_friends
         type: index
possible_keys: NULL
          key: PRIMARY
      key_len: 16
          ref: NULL
         rows: 1316316
        Extra: Using where; Using index; Using temporary
*************************** 2. row ***************************
           id: 1
  select_type: PRIMARY
        table: users
         type: eq_ref
possible_keys: PRIMARY,fk_users_current_location_id
          key: PRIMARY
      key_len: 8
          ref: yoke_int.user_friends.friend_id
         rows: 1
        Extra: Using where; Distinct
*************************** 3. row ***************************
           id: 2
  select_type: DEPENDENT SUBQUERY
        table: user_friends
         type: eq_ref
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 16
          ref: const,func
         rows: 1
        Extra: Using index

Answer 1

There are a couple ways to improve your query.

1. Use a JOIN rather than filterfrom two tables.
2. Rearrange your ANDs so that the simpler ones short circuit faster, stopping the subquery from being ran as often.

Code:

SELECT 
  DISTINCT(friend_id)
FROM 
  user_friends
  JOIN users on users.id = friend_id
WHERE 
  users.id != #{self.id}
AND 
  relationship_status = 'Single'
AND 
  current_location_id IS NOT NULL
AND (
  signed_up IS NOT NULL 
  OR 
  user_id IN (
    SELECT 
      friend_id 
    FROM 
      user_friends 
    WHERE 
      user_id = #{self.id}
  )
) 
;