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From: Samed YILDIRIM <[email protected]>
To: Ankit Kumar Pandey <[email protected]>
Cc: [email protected]
Subject: Re: [QUESTION] Window function with partition by and order by
Date: Sun, 27 Nov 2022 18:23:38 +0200
Message-ID: <CAAo1mb=+SrSf9h7B55yGOu+Q70Suq7oQ0BQ3QuPJmooOM9MkZA@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>

Hello Ankit,

It is absolutely expected behaviour of a window function with ORDER BY
clause. The default frame clause of window definition is *RANGE BETWEEN
UNBOUNDED PRECEDING AND CURRENT ROW*. If you add an ORDER BY clause in a
window definition, PostgreSQL takes the current row and all rows before it
within the partition into calculation. If you don't add, it means all rows
within the partition are peers, and PostgreSQL uses all rows for
calculation. I'm putting the related part from the documentation and its
link below.

The default framing option is RANGE UNBOUNDED PRECEDING, which is the same
> as RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW; it sets the frame to
> be all rows from the partition start up through the current row's last peer
> (a row that the window's ORDER BY clause considers equivalent to the
> current row; all rows are peers if there is no ORDER BY).


https://www.postgresql.org/docs/15/sql-select.html#SQL-WINDOW

Best regards.
Samed YILDIRIM


On Sun, 27 Nov 2022 at 18:08, Ankit Kumar Pandey <[email protected]>
wrote:

> Hello,
>
> While looking at aggregates in window function, I found something
> unusual and would be glad I could get some clarification.
>
> Consider following table (mytable):
>
> id, name
>
> 1, A
>
> 1, A
>
> 2, B
>
> 3, A
>
> 1, A
>
>
> select *, avg(id) over (partition by name, order by id) from mytable;
>
> Output:
>
> id, name, avg
>
> 1, A, 1
>
> 1, A, 1
>
> 1, A, 1
>
> 3, A, 1.5
>
> 2, B, 2
>
>
> Question is: Average of id for partition name (A) should be 6/4 = 1.5
> for all rows in that partition but this result is seen only at the last
> one row in partition (A). Am I missing here something?
>
>
> Thanks
>
>
> --
> Regards,
> Ankit Kumar Pandey
>
>
>
>

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