How to convert timestamp without timezone and integer representing timezone to a new timestamp without timezone in PostgreSQL

Question

Let's say I have a table with two fields: snapshot_date (timestamp without timezone) and timezone (integer),

snapshot_date	timezone
2021-01-12 08:00:00	-3
2021-01-12 00:00:00	-5

I need to extract the local hour from this table, I tried with (EXTRACT (hour from timestamp))::integer + timezone , this would work for the first row, giving an hour result of 5 but with the second row you get -5 , which should actually be 19 since the corresponding date adjusted with timezone is 2021-01-11 19:00:00 . So what I want to do is calculate a new snapshot_date adjusted with the timezone, so I can correctly extract the hour after.

Answer 1

You need to first add the number of hours to the timestamp, then extract the hour:

extract(hour from snapshot_date + make_interval(hours => timezone))

Answer 2

Something like:

show timezone; US/Pacific 
select ('2021-01-12 08:00:00'::timestamp::text || (-3)::text)::timestamptz; 
2021-01-12 03:00:00-08

select extract(hour from('2021-01-12 08:00:00'::timestamp::text || (-3)::text)::timestamptz);
3

Answer 3

Managed to get the expected result with:

extract(hour from timestamp + interval '1 hour' * timezone)