Re: reducing NUMERIC size for 9.1

On 16 July 2010 22:51, Richard Huxton <dev@archonet.com> wrote:
> On 16/07/10 13:44, Brendan Jurd wrote:>
>> At this scale we should be seeing around 2 million bytes saved, but
>> instead the tables are identical.  Is there some kind of disconnect in
>> how the new short numeric is making it to the disk, or perhaps another
>> effect interfering with my test?
>
> You've probably got rows being aligned to a 4-byte boundary. You're probably
> not going to see any change unless you have a couple of 1-byte columns that
> get placed after the numeric. If you went from 10 bytes down to 8, that
> should be visible.

Ah, thanks for the hint Richard.  I didn't see any change with two
1-byte columns after the numeric, but with four such columns I did
finally see a difference.

Test script:

BEGIN;

CREATE TEMP TABLE foo (a numeric, b bool, c bool, d bool, e bool);

INSERT INTO foo (a, b, c, d, e)
SELECT 0::numeric, false, true, i % 2 = 0, i % 2 = 1
FROM generate_series(1, 1000000) i;

SELECT pg_total_relation_size('foo'::regclass);

ROLLBACK;

Results:

8.4: 44326912
HEAD with patch: 36290560

That settles my concern and I'm happy to pass this along to a commiter.

Cheers,
BJ