Subject: Re: Physical column size
From: Ragnar <gnari@hive.is>
To: Paul Mackay <mackaypaul@gmail.com>
Cc: pgsql-general@postgresql.org
In-Reply-To: <786c2f6d0603030203m3b7c62a1k6950ceca066dc5ce@mail.gmail.com>
References: <786c2f6d0601260206k2201a548p48af7c7fc085d3b2@mail.gmail.com>
	<786c2f6d0603030203m3b7c62a1k6950ceca066dc5ce@mail.gmail.com>
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Date: Fri, 03 Mar 2006 10:27:59 +0000
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On fös, 2006-03-03 at 11:03 +0100, Paul Mackay wrote:
> Hi,
> 
> I've created a table like this : 
> CREATE TABLE tmp_A (
> c "char",
> i int4
> );
> 
> And another one 
> CREATE TABLE tmp_B (
> i int4, 
> ii int4
> );
> 
> I then inserted a bit more than 19 million rows in each table (exactly
> the same number of rows in each). 
> 
> The end result is that the physical size on disk used by table tmp_A
> is exactly the same as table tmp_B (as revealed by the
> pg_relation_size function) ! Given that a "char" field is supposed to
> be 1 byte in size and a int4 4 bytes, shouldn't the tmp_A use a
> smaller disk space ? Or is it that any value, whatever the type,
> requires at least 4 bytes to be stored ? 

the int4 needs to be aligned at 4 bytes boundaries,
making wasted space after the char.

this would probably be the same size:

CREATE TABLE tmp_C (
 c "char",
 cc "char",
 i int4
);

and this would be smaller:

CREATE TABLE tmp_D (
 c "char",
 cc "char",
 ccc "char",
);

P.S.: I did not actually check to
see if the "char" type needs to be aligned,
by I assumed not.