SELECT
	ID AS chat_id,
	last_msg,
	user_id
FROM
	(SELECT DISTINCT
			ON (chats. ID) chats. ID,
			chats.last_msg,
			chat_members.user_id
		FROM
			chat_members
		INNER JOIN chats ON chats. ID = chat_members.chat_id
		LEFT JOIN events ON events. ID = chats.event_id
		WHERE (chat_members.user_id = '187'
				OR chats.event_id IN (
					SELECT
						event_id
					FROM
						event_members
					WHERE
						user_id = '187'))
		AND chat_members.date_left IS NULL
		AND chats.event_id IS NOT NULL
		ORDER BY
			chats. ID,
			chats.last_msg,
			chat_members.user_id) AS T
ORDER BY last_msg DESC
LIMIT 5 OFFSET xxx

Limit  (cost=13.93..13.94 rows=5 width=24)
  ->  Sort  (cost=13.93..14.02 rows=34 width=24)
        Sort Key: chats.last_msg
        ->  Unique  (cost=12.86..13.03 rows=34 width=24)
              ->  Sort  (cost=12.86..12.94 rows=34 width=24)
                    Sort Key: chats.id, chats.last_msg, chat_members.user_id
                    ->  Hash Join  (cost=7.41..11.99 rows=34 width=24)
                          Hash Cond: (chat_members.chat_id = chats.id)
                          Join Filter: ((chat_members.user_id = 187::bigint) OR (hashed SubPlan 1))
                          ->  Seq Scan on chat_members  (cost=0.00..3.21 rows=121 width=16)
                                Filter: (date_left IS NULL)
                          ->  Hash  (cost=1.90..1.90 rows=45 width=24)
                                ->  Seq Scan on chats  (cost=0.00..1.90 rows=45 width=24)
                                      Filter: (event_id IS NOT NULL)
                          SubPlan 1
                            ->  Seq Scan on event_members  (cost=0.00..4.90 rows=20 width=8)
                                  Filter: (user_id = 187::bigint)