PostgreSQL - Slow Count - postgresql

Question

I need to write one time query. It will be run one time, and the data will be moved to other system (AWS Personalize). It does not need to be optimized for sure, but at least sped up a bit, so the migration of data is even possible.
Coming from MySQL I thought it would not be a problem. But reading a lot, it seems the COUNT function is handled differently in PostgreSQL. Having mentioned all of that this is the query, reduced in size. There are several other joins (removed from this example), but they do not present an issue, at least looking at the QUERY PLAN.
explain
SELECT DISTINCT ON (p.id)
'plan_progress' AS EVENT_TYPE,
'-1' AS EVENT_VALUE,
extract(EPOCH FROM p.created_at) AS CREATION_TIMESTAMP
FROM plans p
INNER JOIN schedules sch ON p.id = sch.plan_id
WHERE p.status = 'ENDED' AND p.continuous IS NOT TRUE
AND (select Count(id) FROM schedules s WHERE s.plan_id = sch.plan_id AND s.status = 'DONE') = 1
The issue is here:
select Count(id) FROM schedules s WHERE s.plan_id = sch.plan_id AND s.status = 'DONE'
The id field in the schedules table is uuid.
I have tried lots of things, but they all end up the same. Same or worse.
I have read somewhere it is possible to use row estimate in these cases, but I have honestly no idea how to do that in this case.
This is the query plan:
Unique (cost=0.99..25152516038.36 rows=100054 width=88)
-> Nested Loop (cost=0.99..25152515788.22 rows=100054 width=88)
-> Index Only Scan using idx_schedules_plan_id_done_date on schedules sch (cost=0.56..25152152785.84 rows=107641 width=16)
Filter: ((SubPlan 1) = 1)
SubPlan 1
-> Aggregate (cost=1168.28..1168.29 rows=1 width=8)
-> Bitmap Heap Scan on schedules s (cost=14.78..1168.13 rows=58 width=16)
Recheck Cond: (plan_id = sch.plan_id)
Filter: ((status)::text = 'DONE'::text)
-> Bitmap Index Scan on idx_schedules_plan_id_done_date (cost=0.00..14.77 rows=294 width=0)
Index Cond: (plan_id = sch.plan_id)
-> Index Scan using plans_pkey on plans p (cost=0.42..3.37 rows=1 width=24)
Index Cond: (id = sch.plan_id)
Filter: ((continuous IS NOT TRUE) AND ((status)::text = 'ENDED'::text))

Answer 1

you are not selecting any column from the schedules table, so it can be omitted from the main query, and put into an EXISTS() term
distinct is probaly not needed, assuming id is a PK
Maybe you dont need the COUNT() to be exactly one, but just > 0
SELECT DISTINCT ON (p.id)
'plan_progress' AS EVENT_TYPE
, '-1' AS EVENT_VALUE
, extract(EPOCH FROM p.created_at) AS CREATION_TIMESTAMP
FROM plans p
WHERE p.status = 'ENDED' AND p.continuous IS NOT TRUE
AND EXISTS (
SELECT *
FROM schedules sch
WHERE p.id = sch.plan_id
)
AND EXISTS(
select *
FROM schedules s
WHERE s.plan_id = p.id
AND s.status = 'DONE' -- <<-- Must there be EXACTLY ONE schedules record?
) ;
Now you can see that the first EXISTS() is actually not needed: if the second one yields True, the first EXISTS() must yield True, too
SELECT -- DISTINCT ON (p.id)
'plan_progress' AS EVENT_TYPE
, '-1' AS EVENT_VALUE
, extract(EPOCH FROM p.created_at) AS CREATION_TIMESTAMP
FROM plans p
WHERE p.status = 'ENDED' AND p.continuous IS NOT TRUE
AND EXISTS(
select *
FROM schedules s
WHERE s.plan_id = p.id
AND s.status = 'DONE'
) ;