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Message-ID: <71f6fa8c-de7b-4af0-a952-1bd4db4e3007@posteo.de>
Date: Thu, 12 Feb 2026 18:56:30 +0000
MIME-Version: 1.0
Subject: Re: Guarantee order of batched pg_advisory_xact_lock
To: Greg Sabino Mullane <htamfids@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-general@postgresql.org
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From: Nico Heller <nico.heller@posteo.de>
In-Reply-To: <CAKAnmmKfV4K6CfMuQkfQxccP0JRC5orV1tmaq8pB_+EdCZS1yw@mail.gmail.com>
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For my case that is not true as our keys are globally unique URN 
strings. Thus, only the hashes may collide, but thank you for the insight.

On 2/12/26 15:47, Greg Sabino Mullane wrote:
> On Thu, Feb 12, 2026 at 6:18 AM Nico Heller <nico.heller@posteo.de> wrote:
>
>     I just checked for hash collisions with the following query today:
>
>     SELECT COUNT(*), hashtextextended(key, 0) FROM
>     (
>       SELECT key FROM table1
>       UNION
>
> FWIW, you need UNION ALL, not UNION, if you are trying to detect 
> duplicate values (hashed or not) across tables.
>
> Cheers,
> Greg
>
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    <p>For my case that is not true as our keys are globally unique URN
      strings. Thus, only the hashes may collide, but thank you for the
      insight.</p>
    <div class="moz-cite-prefix">On 2/12/26 15:47, Greg Sabino Mullane
      wrote:<br>
    </div>
    <blockquote type="cite"
cite="mid:CAKAnmmKfV4K6CfMuQkfQxccP0JRC5orV1tmaq8pB_+EdCZS1yw@mail.gmail.com">
      <meta http-equiv="content-type" content="text/html; charset=UTF-8">
      <div dir="ltr">
        <div dir="ltr">On Thu, Feb 12, 2026 at 6:18 AM Nico Heller &lt;<a
            href="mailto:nico.heller@posteo.de" moz-do-not-send="true"
            class="moz-txt-link-freetext">nico.heller@posteo.de</a>&gt;
          wrote:</div>
        <div class="gmail_quote gmail_quote_container">
          <blockquote class="gmail_quote"
style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
            <div>
              <p>I just checked for hash collisions with the following
                query today:<br>
                <br>
                SELECT COUNT(*), hashtextextended(key, 0) FROM<br>
                (<br>
                  SELECT key FROM table1<br>
                  UNION </p>
            </div>
          </blockquote>
          <div> </div>
          <div>FWIW, you need UNION ALL, not UNION, if you are trying to
            detect duplicate values (hashed or not) across tables.</div>
          <div><br>
          </div>
          <div>Cheers,</div>
          <div>Greg</div>
          <div><br>
          </div>
        </div>
      </div>
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