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In-Reply-To: <b2202bde6eb145bb814806b5606d131a@alte-leipziger.de>
From: "David G. Johnston" <david.g.johnston@gmail.com>
Date: Sun, 3 May 2026 12:48:47 -0700
Message-ID: <CAKFQuwbRKbjk4JPefgT6s64hN-i47a1nZO6O4j-+h48er6uyvw@mail.gmail.com>
Subject: Re: Trying to understand Tuple Header
To: "Subramanian,Ramachandran" <ramachandran.subramanian@alte-leipziger.de>
Cc: "pgsql-novice@lists.postgresql.org" <pgsql-novice@lists.postgresql.org>
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On Sun, May 3, 2026 at 12:25=E2=80=AFPM Subramanian,Ramachandran <
ramachandran.subramanian@alte-leipziger.de> wrote:

> The binary value of t_infomask for both the tuples are identical, but the=
y
> produce different column values for xmin_commited, xmin_aborted =E2=80=A6=
.  in the
> SQL  !!!.
>
>
>
> Am I not seeing something that is obvious?
>

You didn't re-check the infomask data after running the select query,
instead assuming the bits didn't change.  They did.  SELECT is not a
read-only operation, it participates in optimizations.  Called "writing
hint bits".  Manually evaluating the various tests against the data you did
show would have proven that the pre-select-data had zeros where the second
query claims there are ones.

David J.

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<div dir=3D"ltr"><div dir=3D"ltr"><div class=3D"gmail_default" style=3D"fon=
t-family:arial,helvetica,sans-serif"><span style=3D"font-family:Arial,Helve=
tica,sans-serif">On Sun, May 3, 2026 at 12:25=E2=80=AFPM Subramanian,Ramach=
andran &lt;<a href=3D"mailto:ramachandran.subramanian@alte-leipziger.de">ra=
machandran.subramanian@alte-leipziger.de</a>&gt; wrote:</span></div></div><=
div class=3D"gmail_quote gmail_quote_container"><blockquote class=3D"gmail_=
quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,=
204);padding-left:1ex"><div class=3D"msg-1139417044275290925">


<div lang=3D"DE" style=3D"overflow-wrap: break-word;">
<div class=3D"m_-1139417044275290925WordSection1">
<p class=3D"MsoNormal"><span style=3D"font-family:&quot;Courier New&quot;;f=
ont-size:11pt">The binary value of t_infomask for both the tuples are ident=
ical, but they produce different column values for xmin_commited, xmin_abor=
ted =E2=80=A6. =C2=A0in the SQL
 =C2=A0!!!.</span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11pt;font-family:&quot;Cour=
ier New&quot;"><u></u>=C2=A0<u></u></span></p>
<p class=3D"MsoNormal"><span style=3D"font-size:11pt;font-family:&quot;Cour=
ier New&quot;">Am I not seeing something that is obvious?</span></p></div><=
/div></div></blockquote><div><br></div><div class=3D"gmail_default" style=
=3D"font-family:arial,helvetica,sans-serif">You didn&#39;t re-check the inf=
omask data after running the select query, instead assuming the bits didn&#=
39;t change.=C2=A0 They did.=C2=A0 SELECT is not a read-only operation, it =
participates in optimizations.=C2=A0 Called &quot;writing hint bits&quot;.=
=C2=A0 Manually evaluating the various tests against the data you did show =
would have proven that the pre-select-data had zeros where the second query=
 claims there are ones.</div><div class=3D"gmail_default" style=3D"font-fam=
ily:arial,helvetica,sans-serif"><br></div><div class=3D"gmail_default" styl=
e=3D"font-family:arial,helvetica,sans-serif">David J.</div><div class=3D"gm=
ail_default" style=3D"font-family:arial,helvetica,sans-serif"><br></div><bl=
ockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-lef=
t:1px solid rgb(204,204,204);padding-left:1ex"><div class=3D"msg-1139417044=
275290925"><div lang=3D"DE" style=3D"overflow-wrap: break-word;"><div class=
=3D"m_-1139417044275290925WordSection1"><div>
</div>
</div>
</div>

</div></blockquote></div></div>

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