Does truncate a timestamp break the indexes? - postgresql

Question

I was wondering if my indexes are working well since I am using nodejs and the dates with microseconds are not allowed in this language. So in my query for some good comparison I am doing this kind of thing:
`WHERE (created_at::timestamp(0), uuid) < (${createdAt}::timestamp(0), ${uuid})`
Since I am using a the cast which truncate to seconds, I supposed that the indexes are break. Am I right ? The solution then would be to change the precision of the timestamps stored, or is there another solution to keep the old ones ?

Answer 1

You could change the PostgreSQL data type to millisecond precision:
ALTER TABLE tab ALTER created_at TYPE timestamp(3) without time zone;

Answer 2

By using the recommended EXPLAIN(ANALYZE, VERBOSE, BUFFERS).
I created a table named users with a constraint on the created_at
create table users (
id uuid default uuid_generate_v4() not null
constraint users_pkey primary key,
created_at timestamp default CURRENT_TIMESTAMP
);
create index users_created_at_idx on users (created_at);
The test:
EXPLAIN(ANALYZE, VERBOSE, BUFFERS)
SELECT id
FROM users
WHERE (created_at >= '2022-01-21 15:43:33.631779');
Index Scan using users_created_at_idx on public.users (cost=0.14..4.16 rows=1 width=16) (actual time=0.010..0.018 rows=0 loops=1)
Output: id
Index Cond: (users.created_at >= '2022-01-21 15:43:33.631779'::timestamp without time zone)
Buffers: shared hit=1
Planning Time: 0.074 ms
Execution Time: 0.058 ms
EXPLAIN(ANALYZE, VERBOSE, BUFFERS)
SELECT id
FROM users
WHERE (created_at::timestamp(0) >= '2022-01-21 15:43:33.631779'::timestamp(0));
Seq Scan on public.users (cost=0.00..4.50 rows=33 width=16) (actual time=0.034..0.043 rows=0 loops=1)
Output: id
Filter: ((users.created_at)::timestamp(0) without time zone >= '2022-01-21 15:43:34'::timestamp(0) without time zone)
Rows Removed by Filter: 100
Buffers: shared hit=3
Planning Time: 0.073 ms
Execution Time: 0.089 ms
As we can see the index on the created_at column is not taken into account when we cast and truncate.