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From: David G. Johnston <[email protected]>
To: Durgamahesh Manne <[email protected]>
Cc: pgsql-general <[email protected]>
Subject: Re: Fillfactor effectiveness on existing table
Date: Tue, 10 Feb 2026 08:05:06 -0800
Message-ID: <CAKFQuwYppwZRZQ128T+z638iMNGYcuLGHw42DD_AKSTnZu9QVQ@mail.gmail.com> (raw)
In-Reply-To: <CAJCZko+BFu4c4moqA5kMoJx3776xdYfEsxJQVNu_YYgrBmsCyQ@mail.gmail.com>
References: <CAJCZko+BFu4c4moqA5kMoJx3776xdYfEsxJQVNu_YYgrBmsCyQ@mail.gmail.com>

On Tuesday, February 10, 2026, Durgamahesh Manne <[email protected]>
wrote:

> Hi
>
> I added fillfactor with less than 100 to existing table then ran vacuum
> full to take effect
>
> How to ensure the applied fillfactor is working successfully
>
> A ratio of hot updates in catalog table should higher than value of
> n_dead_tup or n_tup_upd? Or what ?
>
>
While free space on the page is necessary for HOT, it is not sufficient.

If you want to prove fillfactor isn’t buggy I’d suggest contriving a test
case instead inspecting complex real data.  A table with a single bigint
and say 50 fillfactor should be easily visible when inspecting the free
space of a page in the heap (not sure of the exact query for this though).
There is a page-inspect contrib extension that provides low-level details.

David J.

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