Date: Mon, 05 Jun 2023 15:17:17 +0200
From: Tim Schwenke <tim@trallnag.com>
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: "Pgsql Novice" <pgsql-novice@lists.postgresql.org>
Message-ID: <1888bb46d75.c27dbb7f688001.5044215935079224344@trallnag.com>
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 <CAKFQuwamUDxtmA3G4mUP1y4DvntoJ4EA4o02465dfiRO3O9rBQ@mail.gmail.com>
References: <1888b7bb7fa.112313046679561.2788161417267896332@trallnag.com>
 <CAKFQuwamUDxtmA3G4mUP1y4DvntoJ4EA4o02465dfiRO3O9rBQ@mail.gmail.com>
Subject: Re: Does PostgreSQL cache all columns of a table after SELECT?
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Hello David,

from what I understand, in PostgreSQL, tables are stored in one or more fil=
es called segments. There is no separation by columns.

https://www.postgresql.org/docs/8.1/storage.html

This means if I select a single column from a table the first time, the ful=
l file / segment is read and put into page cache if there is enough space. =
This means a table with only one large column large_a takes up less page ca=
che compared to a table with many large columns large_a and large_b, even t=
hough in both cases only large_a is selected.

Is that more or less correct? Ignoring toast?

Tim S.

---- On Mon, 05 Jun 2023 14:58:21 +0200 David G. Johnston  wrote ---

 >=20
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 > On Monday, June 5, 2023, Tim Schwenke tim@trallnag.com> wrote:
 >=20
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 > Does the cache also contain large_b? Or is only large_a cached? Assumpti=
on is that memory is large enough to fit everything.
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 > Shared buffers is a page cache.
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 > David J.=C2=A0
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