Same query, different times - sql

Question

What makes performance vary for the same query? My DB has just ~10 tables and no more than a few thousand rows.
Here's the query:
select
c.first_name,
c.last_name,
c.user_id,
c.photo_url,
s.dialogues
from contributors c
join (
select count(*) dialogues, user_id from (
select contributor_one_uuid user_id from dialogues
union all
select contributor_two_uuid from dialogues
) stats group by user_id
) s on (s.user_id = c.user_id)
where c.visible = true
#1: It takes almost 17 seconds to execute! explain (analyze, buffers) select on the query:
Seq Scan on contributors c (cost=0.00..4205.86 rows=259 width=109) (actual time=0.073..16819.258 rows=260 loops=1)
Filter: visible
Rows Removed by Filter: 13
Buffers: shared hit=3681
SubPlan 1
-> Aggregate (cost=16.06..16.07 rows=1 width=8) (actual time=64.548..64.549 rows=1 loops=260)
Buffers: shared hit=3640
-> Seq Scan on dialogues (cost=0.00..16.05 rows=2 width=0) (actual time=49.155..64.547 rows=1 loops=260)
Filter: ((c.user_id = contributor_one_uuid) OR (c.user_id = contributor_two_uuid))
Rows Removed by Filter: 136
Buffers: shared hit=3640
Planning Time: 0.136 ms
Execution Time: 16819.365 ms
#2. It takes not even a second to execute!
Seq Scan on contributors c (cost=0.00..4205.86 rows=259 width=109) (actual time=0.063..801.278 rows=260 loops=1)
Filter: visible
Rows Removed by Filter: 13
Buffers: shared hit=3681
SubPlan 1
-> Aggregate (cost=16.06..16.07 rows=1 width=8) (actual time=3.080..3.080 rows=1 loops=260)
Buffers: shared hit=3640
-> Seq Scan on dialogues (cost=0.00..16.05 rows=2 width=0) (actual time=0.009..3.079 rows=1 loops=260)
Filter: ((c.user_id = contributor_one_uuid) OR (c.user_id = contributor_two_uuid))
Rows Removed by Filter: 136
Buffers: shared hit=3640
Planning Time: 0.127 ms
Execution Time: 801.379 ms

Answer 1

The engine is performing a SeqScan on contributors and I think that's unavoidable (unless I'm quite mistaken). However, it's also performing a SeqScan on dialogues and this can be prevented with a lateral join.
If the table dialogues has indexes on contributor_one_uuid and also on contributor_two_uuid the query can be rephrased. Hopefully this change can speed it up:
select
c.first_name,
c.last_name,
c.user_id,
c.photo_url,
s.dialogues
from contributors c,
lateral (
select (select count(*) from dialogues d where d.contributor_one_uuid = c.user_id)
+ (select count(*) from dialogues d where d.contributor_two_uuid = c.user_id)
as dialogues
) s on true