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From: "David G. Johnston" <david.g.johnston@gmail.com>
Date: Tue, 22 Oct 2024 16:41:22 -0700
Message-ID: <CAKFQuwa32sJfr+OUrFX8VpfkoNXdRidK93RmLExz_4s-demBrA@mail.gmail.com>
Subject: Re: Unexpected table size usage for small composite arrays
To: Erik Sjoblom <sjoblom65@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@lists.postgresql.org
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On Tue, Oct 22, 2024 at 3:46=E2=80=AFPM Erik Sjoblom <sjoblom65@gmail.com> =
wrote:

>
> Yes, I did expect that the first element should take 24+12 bytes and let'=
s
> round that to 50 bytes.
>

Assuming the 24 is coming from the array overhead you are expecting that
storing a custom composite typed value takes zero overhead.  That is a
faulty assumption.  A user created custom type always takes some overhead
because it is considered a variable structure, even in the case where all
of its fields are fixed-width.  Furthermore, it is self-describing, and so
that description has to go somewhere.  Therefore, there must be a non-zero
per-element overhead to store composite values within an array.  As Tom
told you, the specific non-zero number is 24 bytes.

David J.

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<div dir=3D"ltr"><div dir=3D"ltr"><div class=3D"gmail_default" style=3D"fon=
t-family:arial,helvetica,sans-serif"><span style=3D"font-family:Arial,Helve=
tica,sans-serif">On Tue, Oct 22, 2024 at 3:46=E2=80=AFPM Erik Sjoblom &lt;<=
a href=3D"mailto:sjoblom65@gmail.com">sjoblom65@gmail.com</a>&gt; wrote:</s=
pan></div></div><div class=3D"gmail_quote"><blockquote class=3D"gmail_quote=
" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);=
padding-left:1ex"><div dir=3D"ltr"><div><br><div>Yes, I did expect that the=
 first element should take 24+12 bytes and let&#39;s round that to 50 bytes=
.</div></div></div></blockquote><div><br></div><div class=3D"gmail_default"=
 style=3D"font-family:arial,helvetica,sans-serif">Assuming the 24 is coming=
 from the array overhead you are expecting that storing a custom composite =
typed value takes zero overhead.=C2=A0 That is a faulty assumption.=C2=A0 A=
 user created custom type always takes some overhead because it is consider=
ed a variable structure, even in the case where all of its fields are fixed=
-width.=C2=A0 Furthermore, it is self-describing, and so that description h=
as to go somewhere.=C2=A0 Therefore, there must be a non-zero per-element o=
verhead to store composite values within an array.=C2=A0 As Tom told you, t=
he specific non-zero number is 24 bytes.</div><div class=3D"gmail_default" =
style=3D"font-family:arial,helvetica,sans-serif"><br></div><div class=3D"gm=
ail_default" style=3D"font-family:arial,helvetica,sans-serif">David J.</div=
><div class=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-ser=
if"><br></div></div></div>

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