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From: Jim Nasby <jnasby@upgrade.com>
Date: Thu, 13 Nov 2025 13:30:44 -0600
Message-ID: <CAMFBP2oW2xRBJZjpjtLJ_hjLM5fMMA7Sv9SigK5XA5XKXpYyWA@mail.gmail.com>
Subject: Question about MVCC caveats
To: pgsql-general@postgresql.org
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At [1], the docs state that table rewrite ALTERs result in the relation
appearing empty after the ALTER if another transaction had already taken a
snapshot before the ALTER. A simple test with a repeatable read (or
serializable) transaction confirms this... but is there any other situation
where a snapshot would be taken?

1: https://www.postgresql.org/docs/current/mvcc-caveats.html

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<div dir=3D"ltr">At [1], the docs state that table rewrite ALTERs result in=
 the relation appearing empty after the ALTER if another transaction had al=
ready taken a snapshot before the ALTER. A simple test with a repeatable re=
ad (or serializable) transaction confirms this... but is there any other si=
tuation where a snapshot would be taken?<div><div><br></div><div>1: <a href=
=3D"https://www.postgresql.org/docs/current/mvcc-caveats.html">https://www.=
postgresql.org/docs/current/mvcc-caveats.html</a></div></div></div>

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