From: Tom Lane <tgl@sss.pgh.pa.us>
To: ilsa@ilsadee.com
cc: pgsql-docs@lists.postgresql.org
Subject: Re: What does "Table rewrite" mean?
In-reply-to: <20180119160340.3623.98926@wrigleys.postgresql.org>
References: <20180119160340.3623.98926@wrigleys.postgresql.org>
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 <noreply@postgresql.org>
	message dated "Fri, 19 Jan 2018 16:03:40 +0000"
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Date: Fri, 19 Jan 2018 11:33:43 -0500
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=?utf-8?q?PG_Doc_comments_form?= <noreply@postgresql.org> writes:
> What does a table rewrite do?  Does it drop and recreate the table? 
> Everywhere I look people describe it with fear and trepedation as if it was
> some catastrophically dangerous operation to perform.  What is it?

It means reading the whole table and writing it out in some modified
form (for instance, with some column transformed into a new datatype).
It's not "dangerous" in any way ... but if you've got many GB of data in
the table and you can't afford to have the table locked for a long time,
then it's something to avoid.

			regards, tom lane