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Re: cast bytea to double precision[]

From: Mathieu Dubois <mathieu(dot)dubois(at)limsi(dot)fr>
To: pgsql-novice(at)postgresql(dot)org
Subject: Re: cast bytea to double precision[]
Date: 2011-07-26 07:45:17
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-novice

Le 25/07/2011 17:58, Mathieu Dubois a écrit :
> On 07/25/2011 05:54 PM, Merlin Moncure wrote:
>> On Sun, Jul 24, 2011 at 2:03 PM, Mathieu
>> Dubois<mathieu(dot)dubois(at)limsi(dot)fr> wrote:
>>> I have found a solution by myself for the conversion:
>>> regexp_split_to_array(sig_vector, E',')::double precision[] (elements
>>> are in
>>> fact separated by commas).
>>> To convert my column I have used:
>>> ALTER TABLE my_table ALTER sig_vector TO double precision[] USING
>>> regexp_split_to_array(sig_vector, E',')::double precision[];
>>> Is that correct?
>>> Is it correct to pass the column name to regexp_split_to_array()?
>> Yeah -- you are just passing a column's data into a function as an
>> argument -- standard practice. This will work -- your bytea is really
>> a text column, so it's just a matter of breaking up the string.
>> regexp_* functions are great for that.
> Thank you very much for your reply.
> I will launch the conversion right now.
The main reason to do this was to have smaller backups.
The size of a compressed backup was around 1GB with bytea.
I have converted the columns (on a copy of the database) but the 
expected gain are not here!
With double precision[] it is still around 1GB (a little bit smaller but 
just a few MB).

Also the size on the disk is not smaller.
I have listed the content of /var/lib/postgres/8.4/main/base with du and 
the 2 versions have the same size (3.1GB).

Does it make sense?
My hypothesis is that the compression algorithm is able to find 
regularities the data so it finds the same regularity in bytea and in 
double precision[].

Is there any advantage to use double precision[] over bytea in my case?

> Mathieu
>> merlin

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Subject: Re: cast bytea to double precision[]
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