Ivan> SELECT user1_id,user2_id FROM friend WHERE user1_id=42 OR user2_id=42;
To get friends of user 42:
SELECT user1_id FROM friend WHERE user2_id=42 UNION ALL SELECT user2_id FROM friend WHERE user1_id=42;
assuming you create the (user2_id,user1_id) index, this should get you an Append of two index-only scans. We can use UNION ALL here rather than UNION because the table constraints ensure there are no duplicates.
Thanks! That's a more elegant solution for my query than what I had in mind!