Subject: Re: [HACKERS] Custom compression methods
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru>,
 pgsql-hackers@postgresql.org, Ildar Musin <i.musin@postgrespro.ru>
References: <20171130205155.7mgq2cuqv6zxi25a@alvherre.pgsql>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <6ac2d002-94a7-feab-301d-e790417b43a9@2ndquadrant.com>
Date: Fri, 1 Dec 2017 15:10:23 +0100
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On 11/30/2017 09:51 PM, Alvaro Herrera wrote:
> Tomas Vondra wrote:
> 
>> On 11/30/2017 04:20 PM, Ildus Kurbangaliev wrote:
> 
>>> CREATE COMPRESSION METHOD ts1 FOR tsvector HANDLER
>>> tsvector_compression_handler;
>>
>> Understood. Good to know you've considered it, and I agree it doesn't
>> need to be there from the start (which makes the patch simpler).
> 
> Just passing by, but wouldn't this fit in the ACCESS METHOD group of
> commands?  So this could be simplified down to
> CREATE ACCESS METHOD ts1 TYPE COMPRESSION
> we have that for indexes and there are patches flying for heap storage,
> sequences, etc.  I think that's simpler than trying to invent all new
> commands here.  Then (in a future patch) you can use ALTER TYPE to
> define compression for that type, or even add a column-level option to
> reference a specific compression method.
> 

I think that would conflate two very different concepts. In my mind,
access methods define how rows are stored. Compression methods are an
orthogonal concept, e.g. you can compress a value (using a custom
compression algorithm) and store it in an index (using whatever access
method it's using). So not only access methods operate on rows (while
compression operates on varlena values), but you can combine those two
things together. I don't see how you could do that if both are defined
as "access methods" ...

Furthermore, the "TYPE" in CREATE COMPRESSION method was meant to
restrict the compression algorithm to a particular data type (so, if it
relies on tsvector, you can't apply it to text columns). Which is very
different from "TYPE COMPRESSION" in CREATE ACCESS METHOD.

regards

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