On Sat, 12 Oct 2019 at 17:46, Andrew Gierth <andrew@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>>> "MichaelDBA" == MichaelDBA <MichaelDBA@xxxxxxxxxxx> writes:
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 :)
Thanks!
