From: Tom Lane <tgl@sss.pgh.pa.us>
To: "David G. Johnston" <david.g.johnston@gmail.com>
cc: Ron <ronljohnsonjr@gmail.com>,
        pgsql-admin <pgsql-admin@lists.postgresql.org>
Subject: Re: What to watch out for when ALTERing NUMERIC(38,0) to BIGINT?
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Comments: In-reply-to "David G. Johnston" <david.g.johnston@gmail.com>
	message dated "Thu, 28 Jul 2022 08:19:38 -0700"
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Date: Thu, 28 Jul 2022 11:26:39 -0400
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"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Thu, Jul 28, 2022 at 8:13 AM Ron <ronljohnsonjr@gmail.com> wrote:
>> Besides what's mentioned in
>> https://www.postgresql.org/docs/12/ddl-alter.html#id-1.5.4.8.10, what
>> happens *internally* when I run:
>> ALTER TABLE foo ALTER COLUMN bar TYPE BIGINT;

> IIUC, that would be the silver lining in all of this - the rewritten table
> would have zero fragmentation and bloat.

Yeah.  What happens internally is a table rewrite: the entire content
of the table (and its indexes) is written into a new set of files.
At commit, the old files are deleted.  The main gotchas, for a large
table, are the transient disk space consumption and the fact that the
table stays exclusively locked the whole time.

			regards, tom lane