Re: Question regarding the database page layout.

Hello Greg,

On Tue, Sep 2, 2008 at 8:30 AM, Gregory Stark <stark@enterprisedb.com> wrote:
> "Ryan Bradetich" <rbradetich@gmail.com> writes:
>
>>    4. If require_max_align = true, use the MAXALIGN macro; otherwise
>> use the INTALIGN macro.
>
> Huh, I didn't think of doing it like that.
>
> But I'm confused. You seem to be tweaking the alignment of the data inside the
> tuple? After the tuple header? I thought we had only one byte of wasted space
> in there and that's used by the null bitmap. So unless you have more than 8
> columns and some of them are null I wouldn't expect you to save any space. If
> you do then I guess you could save 4 bytes if the null bitmap is 2-5 bytes
> (mod 8) long.

I was not trying to tweak the alignment of the data inside the tuple
header, I was
trying to adjust the alignment of t_hoff so it would not have the requirement of
MAXALIGN.  I believe my proof-of-concept patch was bad and I need to spend some
more time on it tonight with the new knowledge I gained from this email thread.

> I thought the goal was to save space by aligning the tuples on the page more
> densely. That seems to me to be more fruitful as about half the tuples will
> save four bytes even on tables with small or missing null bitmaps.

That is the goal.  Basically my test data is 44 bytes in length for each tuple.
I have no data in the tuple that is required to be MAXALIGN, but since
t_hoff has
the MAXALIGN requirement I throw away 4 bytes for each tuple (on a
64-bit machine).
This proof-of-concept patch is to help me understand the PostgreSQL code and to
see if I can recover those 4 bytes per tuple.

Thanks again for your feedback!

- Ryan