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Message-ID: <2e8fb31d-b09f-f195-b810-6e6683c656cc@gmail.com>
Date: Sun, 27 Nov 2022 22:25:05 +0530
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Subject: Re: [QUESTION] Window function with partition by and order by
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To: Samed YILDIRIM <samed@reddoc.net>
Cc: pgsql-sql@lists.postgresql.org
References: <7f9f9b1d-b612-d0bd-4674-cfb5a8b2d343@gmail.com>
 <CAAo1mb=+SrSf9h7B55yGOu+Q70Suq7oQ0BQ3QuPJmooOM9MkZA@mail.gmail.com>
From: Ankit Kumar Pandey <itsankitkp@gmail.com>
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 <CAAo1mb=+SrSf9h7B55yGOu+Q70Suq7oQ0BQ3QuPJmooOM9MkZA@mail.gmail.com>
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On 27/11/22 21:53, Samed YILDIRIM wrote:
> 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 
> <itsankitkp@gmail.com> 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
>
>
Okay, I understand this now.

-- 
Regards,
Ankit Kumar Pandey

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    </p>
    <div class="moz-cite-prefix">On 27/11/22 21:53, Samed YILDIRIM
      wrote:<br>
    </div>
    <blockquote type="cite"
cite="mid:CAAo1mb=+SrSf9h7B55yGOu+Q70Suq7oQ0BQ3QuPJmooOM9MkZA@mail.gmail.com">
      <meta http-equiv="content-type" content="text/html; charset=UTF-8">
      <div dir="ltr">
        <div>Hello Ankit,<br>
        </div>
        <div><br>
        </div>
        <div>It is absolutely expected behaviour of a window function
          with ORDER BY clause. The default frame clause of window
          definition is <b><span
              style="background-color:rgb(255,255,255)">RANGE BETWEEN
              UNBOUNDED PRECEDING AND CURRENT ROW</span></b><span
            style="background-color:rgb(255,255,255)">. 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.<br>
          </span></div>
        <div><span style="background-color:rgb(255,255,255)"><br>
          </span></div>
        <blockquote class="gmail_quote" style="margin:0px 0px 0px
          0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">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).</blockquote>
        <div> </div>
        <div><a
            href="https://www.postgresql.org/docs/15/sql-select.html#SQL-WINDOW"
            moz-do-not-send="true" class="moz-txt-link-freetext">https://www.postgresql.org/docs/15/sql-select.html#SQL-WINDOW</a></div>
        <div><br>
        </div>
        <div>
          <div>
            <div dir="ltr" class="gmail_signature"
              data-smartmail="gmail_signature">
              <div dir="ltr">
                <div>Best regards.</div>
                <div>Samed YILDIRIM<br>
                </div>
              </div>
            </div>
          </div>
          <br>
        </div>
      </div>
      <br>
      <div class="gmail_quote">
        <div dir="ltr" class="gmail_attr">On Sun, 27 Nov 2022 at 18:08,
          Ankit Kumar Pandey &lt;<a href="mailto:itsankitkp@gmail.com"
            moz-do-not-send="true" class="moz-txt-link-freetext">itsankitkp@gmail.com</a>&gt;
          wrote:<br>
        </div>
        <blockquote class="gmail_quote" style="margin:0px 0px 0px
          0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hello,<br>
          <br>
          While looking at aggregates in window function, I found
          something <br>
          unusual and would be glad I could get some clarification.<br>
          <br>
          Consider following table (mytable):<br>
          <br>
          id, name<br>
          <br>
          1, A<br>
          <br>
          1, A<br>
          <br>
          2, B<br>
          <br>
          3, A<br>
          <br>
          1, A<br>
          <br>
          <br>
          select *, avg(id) over (partition by name, order by id) from
          mytable;<br>
          <br>
          Output:<br>
          <br>
          id, name, avg<br>
          <br>
          1, A, 1<br>
          <br>
          1, A, 1<br>
          <br>
          1, A, 1<br>
          <br>
          3, A, 1.5<br>
          <br>
          2, B, 2<br>
          <br>
          <br>
          Question is: Average of id for partition name (A) should be
          6/4 = 1.5 <br>
          for all rows in that partition but this result is seen only at
          the last <br>
          one row in partition (A). Am I missing here something?<br>
          <br>
          <br>
          Thanks<br>
          <br>
          <br>
          -- <br>
          Regards,<br>
          Ankit Kumar Pandey<br>
          <br>
          <br>
        </blockquote>
      </div>
    </blockquote>
    <p>Okay, I understand this now.<br>
    </p>
    <pre class="moz-signature" cols="72">-- 
Regards,
Ankit Kumar Pandey</pre>
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