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From: Laurenz Albe <[email protected]>
To: Peter J. Holzer <[email protected]>
To: [email protected]
Subject: Re: Dropping column from big table
Date: Mon, 15 Jul 2024 13:53:25 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
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On Sun, 2024-07-14 at 00:05 +0200, Peter J. Holzer wrote:
> On 2024-07-11 10:06:47 +0200, Laurenz Albe wrote:
> > On Thu, 2024-07-11 at 13:10 +0530, sud wrote:
> > > Dropping will take it's own time for post vacuum however as you
> > > rightly said, it won't be blocking which should be fine. 
> > 
> > I am not certain if you understood this correctly.
> > 
> > Dropping a column is fast, but doesn't reclaim the space.
> > VACUUM won't block anything, but won't reclaim the space.
> > VACUUM (FULL) will block everything, but will also not reclaim the space.
> > 
> > You'd need to use a form of ALTER TABLE that rewrites the table,
> > as indicated in the documentation.
> 
> Unfortunately the documentation indicates very little. It mentions that
> the table will be rewritten with
> 
> * SET ACCESS METHOD
> * a volatile DEFAULT
> * changing the type of an existing column (unless binary coercible)
> 
> All three change something which you probably don't want to change.

Hm, true.

You can always do

  UPDATE tab SET id = id;

followed by

  VACUUM (FULL) tab;

Yours,
Laurenz Albe

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