Re: Make tuple deformation faster

On Tue, 2 Jul 2024 at 02:23, David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
>
> On Mon, 1 Jul 2024 at 23:42, Matthias van de Meent
> <boekewurm+postgres(at)gmail(dot)com> wrote:
> >
> > On Mon, 1 Jul 2024 at 12:49, David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> > >
> > > On Mon, 1 Jul 2024 at 22:07, Matthias van de Meent
> > > <boekewurm+postgres(at)gmail(dot)com> wrote:
> > > > One thing I'm slightly concerned about is that this allocates another
> > > > 8 bytes for each attribute in the tuple descriptor. While that's not a
> > > > lot when compared with the ->attrs array, it's still quite a lot when
> > > > we might not care at all about this data; e.g. in temporary tuple
> > > > descriptors during execution, in intermediate planner nodes.
> > >
> > > I've not done it in the patch, but one way to get some of that back is
> > > to ditch pg_attribute.attcacheoff. There's no need for it after this
> > > patch. That's only 4 out of 8 bytes, however.
> >
> > FormData_pg_attribute as a C struct has 4-byte alignment; AFAIK it
> > doesn't have any fields that require 8-byte alignment? Only on 64-bit
> > systems we align the tuples on pages with 8-byte alignment, but
> > in-memory arrays of the struct wouldn't have to deal with that, AFAIK.
>
> Yeah, 4-byte alignment. "out of 8 bytes" I was talking about is
> sizeof(TupleDescDeformAttr), which I believe is the same "another 8
> bytes" you had mentioned. What I meant was that deleting attcacheoff
> only reduces FormData_pg_attribute by 4 bytes per column and adding
> TupleDescDeformAttr adds 8 per column, so we still use 4 more bytes
> per column with the patch.

I see I was confused, thank you for clarifying. As I said, the
concerns were only small; 4 more bytes only in memory shouldn't matter
much in the grand scheme of things.

> I'm happy to keep going with this version of the patch

+1, go for it.

Kind regards,

Matthias van de Meent
Neon (https://neon.tech)