On 02.11.25 09:05, 'Brian Candler' via Prometheus Users wrote: > Something that's not clear from the documentation of subquery > <https://prometheus.io/docs/prometheus/latest/querying/basics/#subquery> is > how the time is handled. > > For example, take foo[5d:1d]. Are the queries done at > - now > - now - 1 day > - now - 2 day ... etc > > Or are the query times quantized to utime 1d multiples, e.g. > - midnight just gone > - midnight yesterday > - midnight the day before ... etc > > I've been doing some testing with prometheus-2.53.5. Today is 2nd November. > > max_over_time(vector(time())[5d:1d]) # 1762041600 = 2025-11-02 00:00:00 > +0000 > min_over_time(vector(time())[5d:1d]) # 1761696000 = 2025-10-29 00:00:00 > +0000 > > max_over_time(vector(time())[1d:1d]) # 1762041600 > min_over_time(vector(time())[1d:1d]) # 1762041600 > > The latter seems particularly useful, as it gives a way to say "give me the > value of this metric as it was midnight (UTC) just gone", which is > something I've been trying to do. > > My question: is this behaviour intentional, and can be depended on going > forward? (e.g. are their unit tests which guarantee it?) > > Alternatively, is there a better way to get the value of a metric at the > start of a particular time interval?
Sub-queries always align to multiples of the step (in Unix time). This was essentially chosen as a mean to reduce the evaluation cost. However, sometimes you might want to control the alignment. This is what https://github.com/prometheus/prometheus/issues/9767 is about. https://github.com/prometheus/prometheus/pull/17106 is a recent attempt to implement it. (On my review list, but I would welcome other reviewers, as my list is quite long. We had previous attempts that didn't reach the goal.) Circling back to your questions: I do believe that the current alignment is something that we should consider a stable part of PromQL, so you can rely on it. We should also document it more clearly. And we should get https://github.com/prometheus/prometheus/issues/9767 resolved so that people can also do the other thing. -- Björn Rabenstein [PGP-ID] 0x851C3DA17D748D03 [email] [email protected] -- You received this message because you are subscribed to the Google Groups "Prometheus Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/prometheus-users/aROSfK%2BwSCFxPJ17%40mail.rabenste.in.
