Subject: Re: [HACKERS] Custom compression methods
To: Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru>
Cc: pgsql-hackers@postgresql.org, Ildar Musin <i.musin@postgrespro.ru>
References: <20170907194236.4cefce96@wp.localdomain>
 <20170912175505.4afa11fd@wp.localdomain>
 <20171102152836.60c041e4@wp.localdomain>
 <20171114162356.52e3d388@wp.localdomain>
 <62e46a47-08a8-6e06-5e4f-6f52e0a13202@2ndquadrant.com>
 <20171121174717.69ecd8f4@wp.localdomain>
 <bfb3d34e-ba74-02cb-ca2a-43344866d9fd@2ndquadrant.com>
 <20171123123849.5a686d7b@wp.localdomain>
 <57daf28d-ed76-c364-a9ca-65d0ff71a36f@2ndquadrant.com>
 <20171124123800.034c9208@wp.localdomain>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <e3378c06-d805-a3f5-6ea7-af2a2417361e@2ndquadrant.com>
Date: Sat, 25 Nov 2017 02:13:41 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.3.0
MIME-Version: 1.0
In-Reply-To: <20171124123800.034c9208@wp.localdomain>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

Hi,

On 11/24/2017 10:38 AM, Ildus Kurbangaliev wrote:
> ...
> That means compressed datums now in the column with storage
> specified as external. I'm not sure that's a bug or a feature.
>

Interesting. Never realized it behaves like this. Not sure if it's
intentional or not (i.e. bug vs. feature).

> Lets insert them usual way:
> 
>   delete from t2;
>   insert into t2 select repeat(md5(i::text),300) from
>     generate_series(1,100000) s(i);
>   \d+
> 
>                      List of relations
>    Schema | Name | Type  | Owner |  Size   | Description 
>   --------+------+-------+-------+---------+-------------
>    public | t1   | table | ildus | 18 MB   | 
>    public | t2   | table | ildus | 1011 MB | 
> 
> Maybe there should be more common solution like comparison of
> attribute properties?
> 

Maybe, not sure what the right solution is. I just know that if we allow
inserting data into arbitrary tables without recompression, we may end
up with data that can't be decompressed.

I agree that the behavior with extended storage is somewhat similar, but
the important distinction is that while that is surprising the data is
still accessible.

regards

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