Date: Sun, 21 Jan 2018 00:47:50 -0300
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Michael Paquier <michael.paquier@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, ilsa@ilsadee.com,
	pgsql-docs@lists.postgresql.org
Subject: Re: What does "Table rewrite" mean?
Message-ID: <20180121034750.cgfugcqbgnxxm3lg@alvherre.pgsql>
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Michael Paquier wrote:
> On Fri, Jan 19, 2018 at 11:33:43AM -0500, Tom Lane wrote:
> > 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.
> 
> Yeah that can be costly. Note that WAL corresponding to this data needs
> to be generated as well.

Maybe we need to document this somewhere, particularly now that we have
a "table_rewrite" event item.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services