Infinite loop for generate_series with timestamp arguments

Hi all,

I noticed, that the following query will send Postgres into an infinite
loop:

(a)  select generate_series(timestamp '2025-01-30', timestamp
'2025-02-01', interval '1 month -29 days');

Adding one month to 2023-01-30 will result in 2023-02-28 due to February
having no 31st day, and then subtracting 29 days will result in
2025-01-30 again, resulting in a repetition from there. Postgres does
not detect this, and gets caught in an infinite loop. This query is not
the only way this can happen, one can imagine many different setups,
that, at some point, will lead to this problem.

One could certainly argue that “go and do an infinite loop” is plainly
the intended semantics of this query, but in other cases, Postgres tries
to guard against these types of looping generate_series statements:

(b)  select generate_series(timestamp '2025-03-31', timestamp
'2025-04-01', interval '1 month -30 days');
        → ERROR:  step size cannot equal zero

(c)  select generate_series(timestamp '2024-12-31', timestamp
'2025-01-01', interval '1 month -31 days');
        → (0 rows)

It is a bit surprising to me, that all these three cases are handled
differently, as they seem exactly analogous. The issue with all the
expression is that the specified interval, when added to the start date,
results in the start date again.

    select (timestamp '2025-01-30' + interval '1 month -29 days') as A,
      (timestamp '2025-03-31' + interval '1 month -30 days') as B,
      (timestamp '2024-12-31' + interval '1 month -31 days') as C;
        →
                      a          |          b |          c
---------------------+---------------------+---------------------
             2025-01-31 00:00:00 | 2025-03-31 00:00:00 | 2024-12-31
00:00:00

Query (b) seems to be the most straight forward: Postgres seems to flag
the interval '1 month -30 days' as “empty” no matter the context.

    select generate_series(timestamp '2024-12-31', timestamp
'2025-01-01', interval '1 month -30 days');
        → psql:commands.sql:1: ERROR:  step size cannot equal zero
    select timestamp '2024-12-31' + interval '1 month -30 days';
        → 2025-01-01 00:00:00

This should theoretically be an unproblematic query resulting in
[2024-12-31 00:00:00, 2025-01-01 00:00:00], but it seems like Postgres
treats '1 month -30 days' as an “zero-size interval”, despite there
being contexts, where it is not of size zero, and contexts, where other
intervals that aren’t flagged, are of size zero.

For query (c), the empty result makes it seem like Postgres interprets
it as having a negative step size for a date in the future; the same
result would happen with e. g. the interval '1 month -40 days' or
generate_series(10, 5, +1). This would fit with the above, where
Postgres assumes that a month always has 30 days for the purpose of
determining how generate_series should run.

Is this behavior intentional or should it be considered to be erroneous?
Naturally, working with human time representation can introduce many
subtleties and maybe it is beyond the scope of Postgres to correctly
handle them (whatever that means), but at least failing in a consistent
way shouldn’t be to much to ask for? If this is intentional, is there a
system to this that I have failed to see?

Best,
Jakob

(PS: as this is not really a bug report, but rather an inquiry to what
extent this behavior is intended, I hope `postgres-general` is the
correct choice of lists. If not, feel free to redirect me to a more
appropriate one.)

---
Jakob Teuber
jakob(dot)teuber(at)tum(dot)de