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From: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Date: Mon, 25 Aug 2025 15:32:03 +0200
Message-ID: 
 <CADzfLwUqZ_s9FzonE-z0REOm8Q0aDVJh_W8S1r9vTYSwBLHo+g@mail.gmail.com>
Subject: Re: [BUG?] check_exclusion_or_unique_constraint false negative
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>,
 Peter Geoghegan <pg@bowt.ie>,
	"Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
 Andres Freund <andres@anarazel.de>
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Amit Kapila <amit.kapila16@gmail.com>:


> What if the new insert happens in a page prior to the current page? I
> mean that the scan won't encounter the page where Insert happens.

Hmm.... Yes - if the TID lands to the page left of the current
position, we=E2=80=99ll miss it as well.
A lock=E2=80=91based solution (version in the v10) would require keeping al=
l
pages with the same key under a read lock, which feels too expensive.

> BTW, do we know the reason behind using SnapshotDirty in the first
> place? I don't see any comments in the nearby code unless I am missing
> something.

I think this is simply an attempt to lock the newest version of the
logical tuple, including INSERT cases.
For an existing tuple, the same can be achieved using MVCC snapshot + retry=
.
However, in the case of a not-yet-committed INSERT, a different type
of snapshot is required.

But I'm not sure if it provides any advantages.