Subject: Re: [HACKERS] Custom compression methods
To: Robert Haas <robertmhaas@gmail.com>,
 Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru>
Cc: Oleg Bartunov <obartunov@gmail.com>, Craig Ringer
 <craig@2ndquadrant.com>, Peter Eisentraut
 <peter.eisentraut@2ndquadrant.com>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
 Andres Freund <andres@anarazel.de>
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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <e13abfa9-658d-8bef-4a69-68ed4ea34ee9@2ndquadrant.com>
Date: Mon, 20 Nov 2017 00:23:23 +0100
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On 11/15/2017 02:13 PM, Robert Haas wrote:
> On Wed, Nov 15, 2017 at 4:09 AM, Ildus Kurbangaliev
> <i.kurbangaliev@postgrespro.ru> wrote:
>> So in the next version of the patch I can just unlink the options from
>> compression methods and dropping compression method will not affect
>> already compressed tuples. They still could be decompressed.
> 
> I guess I don't understand how that can work.  I mean, if somebody
> removes a compression method - i.e. uninstalls the library - and you
> don't have a way to make sure there are no tuples that can only be
> uncompressed by that library - then you've broken the database.
> Ideally, there should be a way to add a new compression method via an
> extension ... and then get rid of it and all dependencies thereupon.
> 

I share your confusion. Once you do DROP COMPRESSION METHOD, there must
be no remaining data compressed with it. But that's what the patch is
doing already - it enforces this using dependencies, as usual.

Ildus, can you explain what you meant? How could the data still be
decompressed after DROP COMPRESSION METHOD, and possibly after removing
the .so library?

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services