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From: Romain Carl <[email protected]>
To: Tom Lane <[email protected]>
Cc: [email protected]
Subject: Re: Window functions: frame-adhering aggregate without ORDER BY clause
Date: Mon, 26 Jun 2023 17:14:15 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<[email protected]>

Alright, this makes sense. Thank you for the quick response!

Best regards,
Romain Carl

On 26.06.23 15:54, Tom Lane wrote:
> Romain Carl <[email protected]> writes:
>> among the window tests (src/test/regress/expected/window.out), I noticed
>> the presence of tests that rely upon the order of rows not determined by
>> any ORDER BY clause, such as:
> Yeah ...
>
>> The current row's frame and, consequently, the result of the sum
>> aggregate depend on the order produced by the sequential scan of table
>> tenk1. Since such order is, in general, not part of PG's defined
>> behavior, what purpose do the tests that rely upon it serve?
> The tests are perfectly entitled to test PG's actual behavior.
> I don't see much difference between this particular case and the
> fact that we have any tests at all that lack ORDER BY, because
> formally speaking the engine could choose to emit the rows in
> some other order.  In practice, if we ever did make the engine
> behave differently, it'd be on us to fix affected test cases.
>
>> Following up to that, how is an EXCLUDE GROUP defined to behave in
>> absence of any ORDER BY clause?
> I see in the docs
>
>      <literal>EXCLUDE GROUP</literal> excludes the current row and its
>      ordering peers from the frame.
>
> and a bit later
>
>      Without <literal>ORDER BY</literal>,
>      ... all rows become peers of the current row.
>
> so excluding the whole frame seems like the right behavior.
>
> 			regards, tom lane

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