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help / color / mirror / Atom feedFrom: David G. Johnston <david.g.johnston@gmail.com>
To: Subramanian,Ramachandran <ramachandran.subramanian@alte-leipziger.de>
Cc: pgsql-novice@lists.postgresql.org <pgsql-novice@lists.postgresql.org>
Subject: Re: Trying to understand Tuple Header
Date: Sun, 3 May 2026 12:48:47 -0700
Message-ID: <CAKFQuwbRKbjk4JPefgT6s64hN-i47a1nZO6O4j-+h48er6uyvw@mail.gmail.com> (raw)
In-Reply-To: <b2202bde6eb145bb814806b5606d131a@alte-leipziger.de>
References: <0f8e5544ae8a412bb637fcc3d15f8aaa@alte-leipziger.de>
<CAKFQuwawOYYR=Dfa3tWsKOd-baJ-hV7SKMj9FHiMnd9f-=vNAg@mail.gmail.com>
<b2202bde6eb145bb814806b5606d131a@alte-leipziger.de>
On Sun, May 3, 2026 at 12:25 PM Subramanian,Ramachandran <
ramachandran.subramanian@alte-leipziger.de> wrote:
> The binary value of t_infomask for both the tuples are identical, but they
> produce different column values for xmin_commited, xmin_aborted …. 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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To: pgsql-novice@postgresql.org
Cc: david.g.johnston@gmail.com, ramachandran.subramanian@alte-leipziger.de, pgsql-novice@lists.postgresql.org
Subject: Re: Trying to understand Tuple Header
In-Reply-To: <CAKFQuwbRKbjk4JPefgT6s64hN-i47a1nZO6O4j-+h48er6uyvw@mail.gmail.com>
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