MichaelDBA> Nope, vacuumed it and still got the bitmap index scans.
Let's see your explains. Here's mine:
# set enable_seqscan=false; -- because I only have a few rows SET # insert into friend values (1,2),(2,5); INSERT 0 2 # vacuum analyze friend; VACUUM # explain analyze SELECT user1_id FROM friend WHERE user2_id=2 UNION ALL select user2_id FROM friend WHERE user1_id=2; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------------------ Append (cost=0.13..8.32 rows=2 width=4) (actual time=0.009..0.014 rows=2 loops=1) -> Index Only Scan using friend_user2_id_user1_id_idx on friend (cost=0.13..4.15 rows=1 width=4) (actual time=0.009..0.009 rows=1 loops=1) Index Cond: (user2_id = 2) Heap Fetches: 0 -> Index Only Scan using friend_pkey on friend friend_1 (cost=0.13..4.15 rows=1 width=4) (actual time=0.003..0.004 rows=1 loops=1) Index Cond: (user1_id = 2) Heap Fetches: 0 Planning Time: 0.271 ms Execution Time: 0.045 ms (9 rows)
Note that you have to put some actual rows in the table; if it is completely empty, you'll not get a representative result.
Confirming what's been said - the whole thing works fine on 10. I can't get index only scans on 9.6, but that's a dev machine anyway.
Now if only hash indexes supported multiple column, that'd probably result in all my data being returned from a single read of a hash bucket, but that's going into microoptimisation territory :)
