On Sun, May 19, 2024 at 2:20 AM Przemysław Sztoch <przemys...@sztoch.pl>
wrote:
> Robert Haas wrote on 5/15/2024 9:29 PM:
>
> On Mon, Mar 4, 2024 at 5:03 AM Przemysław Sztoch <przemys...@sztoch.pl>
> <przemys...@sztoch.pl> wrote:
>
> Apparently the functionality is identical to date_bin.
> When I saw date_bin in the documentation, I thought it solved all my problems.
> Unfortunately, DST problems have many corner cases.
> I tried to change date_bin several times, but unfortunately in some cases it
> would start working differently than before.
>
> So, first of all, thanks for taking an interest and sending a patch.
>
> In order for the patch to have a chance of being accepted, we would
> need to have a clear understanding of exactly how this patch is
> different from the existing date_bin(). If we knew that, we could
> decide either that (a) date_bin does the right thing and your patch
> does the wrong thing and therefore we should reject your patch, or we
> could decide that (b) date_bin does the wrong thing and therefore we
> should fix it, or we could decide that (c) both date_bin and what this
> patch does are correct, in the sense of being sensible things to do,
> and there is a reason to have both. But if we don't really understand
> how they are different, which seems to be the case right now, then we
> can't make any decisions. And what that means in practice is that
> nobody is going to be willing to commit anything, and we're just going
> to go around in circles.
>
> Typically, this kind of research is the responsibility of the patch
> author: you're the one who wants something changed, so that means you
> need to provide convincing evidence that it should be. If someone else
> volunteers to do it, that's also cool, but it absolutely has to be
> done in order for there to be a chance of progress here. No committer
> is going to say "well, we already have date_bin, but Przemysław says
> his date_trunc is different somehow, so let's have both without
> understanding how exactly they're different." That's just not a
> realistic scenario. Just to name one problem, how would we document
> each of them? Users would expect the documentation to explain how two
> closely-related functions differ, but we will be unable to explain
> that if we don't know the answer ourselves.
>
> If you can't figure out exactly what the differences are by code
> inspection, then maybe one thing you could do to help unblock things
> here is provide some very clear examples of when they deliver the same
> results and when they deliver different results. Although there are no
> guarantees, that might lead somebody else to jump in and suggest an
> explanation, or further avenues of analysis, or some other helpful
> comment.
>
> Personally, what I suspect is that there's already a way to do what
> you want using date_bin(), maybe in conjunction with some casting or
> some calls to other functions that we already have. But it's hard to
> be sure because we just don't have the details. "DST problems have
> many corner cases" and "in some cases [date_bin] would start working
> differently than before" may be true statements as far as they go, but
> they're not very specific complaints. If you can describe *exactly*
> how date_bin fails to meet your expectations, there is a much better
> chance that something useful will happen here.
>
>
> I would also like to thank Robert for presenting the matter in detail.
>
> My function date_trunc ( interval, timestamp, ...) is similar to original
> function date_trunc ( text, timestamp ...) .
>
> My extension only gives more granularity.
> We don't have a jump from hour to day. We can use 6h and 12h. It's the
> same with minutes.
> We can round to 30 minutes, 20 minutes, 15 minutes, etc.
>
> Using date_bin has a similar effect, but requires specifying the origin.
> According to this origin,
> subsequent buckets are then calculated. The need to provide this origin is
> sometimes a very big problem.
> Especially since you cannot use one origin when changing from summer to
> winter time.
>
> If we use one origin for example begin of year: 2024-01-01 00:00:00 then:
> # SET timezone='Europe/Warsaw';
> # SELECT date_bin('1 day', '2024-03-05 11:22:33', '2024-01-01 00:00:00'),
> date_trunc('day', '2024-03-05 11:22:33'::timestamptz);
> 2024-03-05 00:00:00+01 2024-03-05 00:00:00+01 date_bin works ok,
> because we are before DST
> # SELECT date_bin('1 day', '2024-05-05 11:22:33', '2024-01-01 00:00:00'),
> date_trunc('day', '2024-05-05 11:22:33'::timestamptz);
> 2024-05-05 01:00:00+02 2024-05-05 00:00:00+02 date_bin has
> problem, because we are in May after DST
>
> If anyone has an idea how to make date_bin work like date_trunc, please
> provide an example.
>
>
Here is an example which will make date_bin() to behave like date_trunc():
# SELECT date_bin('1 day', '2024-05-05 11:22:33', '0001-01-01'::timestamp),
date_trunc('day', '2024-05-05 11:22:33'::timestamptz);
date_bin | date_trunc
---------------------+------------------------
2024-05-05 00:00:00 | 2024-05-05 00:00:00+02
(1 row)
In general, to make date_bin work similarly to date_trunc in PostgreSQL,
you need to set the interval length appropriately and use an origin
timestamp that aligns with the start of the interval you want to bin.
Here's how you can use date_bin to mimic the behavior of date_trunc:
Truncate to the Start of the Year:
# SELECT date_bin('1 year', timestamp_column, '0001-01-01'::timestamp) FROM
your_table;
Truncate to the Start of the Month:
# SELECT date_bin('1 month', timestamp_column, '0001-01-01'::timestamp)
FROM your_table;
Truncate to the Start of the Day:
# SELECT date_bin('1 day', timestamp_column, '0001-01-01'::timestamp) FROM
your_table;
Truncate to the Start of the Hour:
# SELECT date_bin('1 hour', timestamp_column, '0001-01-01'::timestamp) FROM
your_table;
Truncate to the Start of the Minute:
# SELECT date_bin('1 minute', timestamp_column, '0001-01-01'::timestamp)
FROM your_table;
--
> Przemysław Sztoch | Mobile +48 509 99 00 66
>
