Re: Question regarding the database page layout.

Hello Greg,

On Mon, Aug 11, 2008 at 2:24 AM, Gregory Stark <stark(at)enterprisedb(dot)com>wrote:

> "Ryan Bradetich" <rbradetich(at)gmail(dot)com> writes:
>
> > After a cursory glance at the HeapTupleHeaderData structure, it appears
> it
> > could be aligned with INTALIGN instead of MAXALIGN. The one structure I
> was
> > worried about was the 6 byte t_ctid structure. The comments in
> > src/include/storage/itemptr.h file indicate the ItemPointerData structure
> is
> > composed of 3 int16 fields. So everthing in the HeapTupleHeaderData
> > structure is 32-bits or less.
>
> Sure, but the tuple itself could contain something with double alignment.
> If
> you have a bigint or double in the tuple then heap_form_tuple needs to know
> where to put it so it ends up at right alignment.

My first thought was we can still figure this out because the user data is
already
forced to be MAXALIGN. Then I realized oh, since that is true then I am
still going
to eat the padding anyhow. The padding would just move to one of two
places:
1. To MAXALIGN the user data or
2. To MAXALIGN the heap tuple header.

Thanks for the sanity check. I was wrapped up in looking for alignment
requirements
in the HeapTupleHeaderData structure, I overlooked the significance of the
MAXALIGN
on the user data.

Thanks,

- Ryan

P.S. Just for the archive, it seems this idea still may be workable (need
to look at the
heap_form_tuple in significant more detail) if/when someone implements the
proposal
to separate the physical storage from the column order. That solution is a
bit more
than I am ready to tackle at the moment :) Maybe in the future.