Postgres time comparaison with time zone - postgresql

Question

Today I encounter a strange postgres behavious. Let me explain:
Here is my table I will work on.
=># \d planning_time_slot
Table "public.planning_time_slot"
Column | Type | Collation | Nullable | Default
-------------+---------------------------+-----------+----------+------------------------------------------------
id | integer | | not null | nextval('planning_time_slot_id_seq'::regclass)
planning_id | integer | | not null |
day | character varying(255) | | not null |
start_time | time(0) without time zone | | not null |
end_time | time(0) without time zone | | not null |
day_id | integer | | not null | 0
Indexes:
"planning_time_slot_pkey" PRIMARY KEY, btree (id)
"idx_a9e3f3493d865311" btree (planning_id)
Foreign-key constraints:
"fk_a9e3f3493d865311" FOREIGN KEY (planning_id) REFERENCES planning(id)
what i want to do is something like:
select * from planning_time_slot where start_time > (CURRENT_TIME AT TIME ZONE 'Europe/Paris');
But it seems like postgres is comparing time before the time zone conversion.
Here is my tests:
=># select * from planning_time_slot where start_time > (CURRENT_TIME AT TIME ZONE 'Europe/Paris');
id | planning_id | day | start_time | end_time | day_id
-----+-------------+-----+------------+----------+--------
157 | 6 | su | 16:00:00 | 16:30:00 | 0
(1 row)
=># select (CURRENT_TIME AT TIME ZONE 'Europe/Paris');
timezone
--------------------
16:35:48.591002+02
(1 row)
When I try with a lot of entries it appears that the comparaison is done between start_time and CURRENT_TIME without the time zone cast.
For your information I also tried :
select * from planning_time_slot where start_time > timezone('Europe/Paris', CURRENT_TIME);
It has the exact same result.
I also tried to change the column type to time(0) with time zone. It makes the exact same result.
One last important point. I really need to set timezone I want, because later on I will change it dynamically depending on other stuffs. So it will not be 'Europe/Paris' everytime.
Does anyone have a clue or a hint please ?
psql (PostgreSQL) 11.2 (Debian 11.2-1.pgdg90+1)

Answer 1

(CURRENT_TIME AT TIME ZONE 'Europe/Paris') is, for example, 17:52:17.872082+02. But internally it is 15:52:17.872082+00. Both time and timetz (time with time zone) are all stored as UTC, the only difference is timetz is stored with a time zone. Changing the time zone does not change what point in time it represents.
So when you compare it with a time...
# select '17:00:00'::time < '17:52:17+02'::timetz;
?column?
----------
f
That is really...
# select '17:00:00'::time < '15:52:17'::time;
?column?
----------
f
Casting a timetz to a time will lop off the time zone.
test=# select (CURRENT_TIME AT TIME ZONE 'Europe/Paris')::time;
timezone
-----------------
17:55:57.099863
(1 row)
test=# select '17:00:00' < (CURRENT_TIME AT TIME ZONE 'Europe/Paris')::time;
?column?
----------
t
Note that this sort of comparison only makes sense if you want to store the notion that a thing happens at 17:00 according to the clock on the wall. For example, if you had a mobile phone game where an event starts "at 17:00" meaning 17:00 where the user is. This is referred to as a "floating time zone".
Assuming day is "day of week", I suggest storing it as an integer. It's easier to compare and localize.
Instead of separate start and end times, consider a single timerange. Then you can use range operators.

Answer 2

I think you have deeper problems.
You have a day, a start time and an end time, but no notion of time zone. So this will mean something different depending on the time zone of the observer.
I think you should add a tz column that stores which time zone that information is in. Then you can get the start time like this:
WHERE (day + start_time) AT TIME ZONE tz > current_timestamp