SQL relative versus absolute date impact query time - sql

Question

I run the following query and it takes 50 seconds.
select created_at, currency, balance
from YYY
where id in (ZZZ) and currency = 'XXX'
and created_at >= '2020-08-28'
order by created_at desc
limit 1;
explain:
Limit (cost=100.12..1439.97 rows=1 width=72)
-> Foreign Scan on yyy (cost=100.12..21537.65 rows=16 width=72)
Filter: (("substring"((object_key)::text, '\w+:(\d+):.*'::text))::integer = 723120)
Then I run the following query and it "infinite" time. Too long to wait until the end.
select created_at, currency, balance
from YYY
where id in (ZZZ) and currency = 'XXX'
and created_at >= NOW() - INTERVAL '1 DAY'
order by created_at desc
limit 1;
explain:
Limit (cost=53293831.90..53293831.91 rows=1 width=72)
-> Result (cost=53293831.90..53293987.46 rows=17284 width=72)
-> Sort (cost=53293831.90..53293840.54 rows=17284 width=556)
Sort Key: yyy.created_at DESC
-> Foreign Scan on yyy (cost=100.00..53293814.62 rows=17284 width=556)
Filter: ((created_at >= (now() - '1 day'::interval)) AND (("substring"((object_key)::text, '\w+:(\d+):.*'::text))::integer = 723120))
What could make this huge difference between those query. I know that index are used to improve performance. What can we infer from here?
Any contribution would be appreciated.

Answer 1

With a literal, the optimizer has an easy game to plan an efficient data access using the right index.
With an expression like NOW - INTERVAL '4 DAY', you run at least into two challenges:
It is a stable, not an immutable, expression. Let alone a literal.
The expression is a TIMESTAMP WITH TIME ZONE, not a DATE, and you need an implicit type cast.
You just make the life of the optimizer difficult ...
I just created a single-column table yyy with 12 years' worth of distinct dates in my PostgreSQL database. No indexes. Already here, you see a difference in the cost of the explain plan.
$ psql -c "explain select * from yyy where created_at >= '2020-08-28'"
QUERY PLAN
------------------------------------------------------
Seq Scan on yyy (cost=0.00..74.79 rows=126 width=4)
Filter: (created_at >= '2020-08-28'::date)
And:
$ psql -c "explain select * from yyy where created_at >= now() - interval '4 day'"
QUERY PLAN
--------------------------------------------------------
Seq Scan on yyy (cost=0.00..96.70 rows=126 width=4)
Filter: (created_at >= (now() - '4 days'::interval))
(2 rows)
It will be a much worse difference with the existence of an index ....