Re: Physical column size

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.