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From: Dominique Devienne <ddevienne@gmail.com>
Date: Thu, 20 Feb 2025 15:33:57 +0100
Message-ID: <CAFCRh--3iD9sSwakKvm9-Od+0RBEuLGy35dD72C5G9PzCYEiTg@mail.gmail.com>
Subject: Re: v18 virtual columns
To: Ron Johnson <ronljohnsonjr@gmail.com>
Cc: "pgsql-generallists.postgresql.org" <pgsql-general@lists.postgresql.org>
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On Thu, Feb 20, 2025 at 3:31=E2=80=AFPM Ron Johnson <ronljohnsonjr@gmail.co=
m> wrote:

> On Thu, Feb 20, 2025 at 8:07=E2=80=AFAM Dominique Devienne <ddevienne@gma=
il.com>
> wrote:
>
>> Hi. I've just read
>> https://www.dbi-services.com/blog/postgresql-18-virtual-generated-column=
s/
>>
>> and I'm wondering whether there will be a way to ALTER existing STORED
>> generated columns, to be virtual? W/o rewriting the whole table that is.
>>
>
> What about dropping the existing stored column then adding the new virtua=
l
> column?
>
> You won't get any disk space saving for existing records, but it's not
> going to rewrite the table.
>

Didn't think of that, but yes, I think that would work.
They are GENERATED columns, so the values stored are derived data anyway,
indeed.

I also didn't know DROP COLUMN was smart enough not rewriting the column,
that's good.

Thanks, --DD

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<div dir=3D"ltr"><div dir=3D"ltr">On Thu, Feb 20, 2025 at 3:31=E2=80=AFPM R=
on Johnson &lt;<a href=3D"mailto:ronljohnsonjr@gmail.com">ronljohnsonjr@gma=
il.com</a>&gt; wrote:</div><div class=3D"gmail_quote gmail_quote_container"=
><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 di=
r=3D"ltr">On Thu, Feb 20, 2025 at 8:07=E2=80=AFAM Dominique Devienne &lt;<a=
 href=3D"mailto:ddevienne@gmail.com" target=3D"_blank">ddevienne@gmail.com<=
/a>&gt; wrote:</div><div class=3D"gmail_quote"><blockquote class=3D"gmail_q=
uote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,2=
04);padding-left:1ex"><div dir=3D"ltr">Hi. I&#39;ve just read=C2=A0<a href=
=3D"https://www.dbi-services.com/blog/postgresql-18-virtual-generated-colum=
ns/" target=3D"_blank">https://www.dbi-services.com/blog/postgresql-18-virt=
ual-generated-columns/</a><div><br></div><div>and I&#39;m wondering whether=
 there will be a way to ALTER existing STORED generated columns, to be virt=
ual? W/o rewriting the whole table that is.</div></div></blockquote><div><b=
r></div><div>What about dropping the existing stored column then adding the=
 new virtual column?</div><div><br></div><div>You won&#39;t get any disk sp=
ace saving for existing records, but it&#39;s not going to rewrite the tabl=
e.</div></div></div></blockquote><div><br></div><div>Didn&#39;t think of th=
at, but yes, I think that would work.</div><div>They are GENERATED columns,=
 so the values stored are derived data anyway, indeed.</div><div><br></div>=
<div>I also didn&#39;t know DROP COLUMN was smart enough not rewriting the =
column, that&#39;s good.</div><div><br></div><div>Thanks, --DD</div></div><=
/div>

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