Re: Optimize SnapBuildPurgeOlderTxn: use in-place compaction instead of temporary array

From: Neil Chen <carpenter(dot)nail(dot)cz(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Xuneng Zhou <xunengzhou(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Kirill Reshke <reshkekirill(at)gmail(dot)com>
Subject: Re: Optimize SnapBuildPurgeOlderTxn: use in-place compaction instead of temporary array
Date: 2026-01-09 02:17:55
Message-ID: CAA3qoJmiYyHykgVW7LA21-31N_+_NS0iJX93PnCCmjNQ5RKUHg@mail.gmail.com
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Hi Tom,

On Fri, Jan 9, 2026 at 12:05 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

>
> What makes you think this code isn't adequately tested already?
> The coverage report at
>
>
> https://coverage.postgresql.org/src/backend/replication/logical/snapbuild.c.gcov.html
>
> shows SnapBuildPurgeOlderTxn as pretty fully exercised.
>
>
I wasn’t aware of this website before, so thank you for sharing it.
Actually, this patch evolved from a tiny, "casual" quick-fix patch in its
very first version. I agree that the current effort invested in it possible
has outweighed the potential benefits it may bring.

On a side note, I’m a beginner with PostgreSQL and trying to take on some
simple tasks while deepening my understanding of the system. I noticed that
many items in the coverage tests you provided have rather low coverage
rates (< 75%). Do you think it would be worthwhile to add more test cases
to improve their test coverage? I’d appreciate any advice the community can
offer on this.

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