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From: sud <suds1434@gmail.com>
Date: Thu, 11 Jul 2024 08:58:18 +0530
Message-ID: <CAD=mzVUS8mFK42zmVUUC3JkiN=9QSUZ_Qxayd6=KJwbqdd0HzA@mail.gmail.com>
Subject: Re: Dropping column from big table
To: Adrian Klaver <adrian.klaver@aklaver.com>
Cc: pgsql-general <pgsql-general@lists.postgresql.org>
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On Thu, Jul 11, 2024 at 2:52=E2=80=AFAM Adrian Klaver <adrian.klaver@aklave=
r.com>
wrote:

>
> https://www.postgresql.org/docs/current/sql-altertable.html
>
> "The DROP COLUMN form does not physically remove the column, but simply
> makes it invisible to SQL operations. Subsequent insert and update
> operations in the table will store a null value for the column. Thus,
> dropping a column is quick but it will not immediately reduce the
> on-disk size of your table, as the space occupied by the dropped column
> is not reclaimed. The space will be reclaimed over time as existing rows
> are updated.
>
> To force immediate reclamation of space occupied by a dropped column,
> you can execute one of the forms of ALTER TABLE that performs a rewrite
> of the whole table. This results in reconstructing each row with the
> dropped column replaced by a null value.
> "
>
>
Thank you so much. When you said *"you can execute one of the forms of
ALTER TABLE that performs a rewrite*
*of the whole table."* Does it mean that post "alter table drop column" the
vacuum is going to run longer as it will try to clean up all the rows and
recreate the new rows? But then how can this be avoidable or made better
without impacting the system performance and blocking others?

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<div dir=3D"ltr"><div dir=3D"ltr"><br></div><div class=3D"gmail_quote"><div=
 dir=3D"ltr" class=3D"gmail_attr">On Thu, Jul 11, 2024 at 2:52=E2=80=AFAM A=
drian Klaver &lt;<a href=3D"mailto:adrian.klaver@aklaver.com">adrian.klaver=
@aklaver.com</a>&gt; wrote:<br></div><blockquote class=3D"gmail_quote" styl=
e=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);paddin=
g-left:1ex"><br>
<a href=3D"https://www.postgresql.org/docs/current/sql-altertable.html" rel=
=3D"noreferrer" target=3D"_blank">https://www.postgresql.org/docs/current/s=
ql-altertable.html</a><br>
<br>
&quot;The DROP COLUMN form does not physically remove the column, but simpl=
y <br>
makes it invisible to SQL operations. Subsequent insert and update <br>
operations in the table will store a null value for the column. Thus, <br>
dropping a column is quick but it will not immediately reduce the <br>
on-disk size of your table, as the space occupied by the dropped column <br=
>
is not reclaimed. The space will be reclaimed over time as existing rows <b=
r>
are updated.<br>
<br>
To force immediate reclamation of space occupied by a dropped column, <br>
you can execute one of the forms of ALTER TABLE that performs a rewrite <br=
>
of the whole table. This results in reconstructing each row with the <br>
dropped column replaced by a null value.<br>
&quot;<br><br></blockquote><div><br></div><div>Thank you so much. When you =
said <i>&quot;you can execute one of the forms of ALTER TABLE that performs=
 a rewrite</i></div><div><i>of the whole table.&quot;</i>=C2=A0Does it mean=
 that post &quot;alter table drop column&quot; the vacuum is going to run l=
onger as it will try to clean up all the rows and recreate the new rows? Bu=
t then how can this be avoidable or made better without impacting the syste=
m performance and=C2=A0blocking others?</div></div></div>

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