Re: making relfilenodes 56 bits

On Fri, Jun 24, 2022 at 9:30 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> If we "inline" RelFileNumber, it's probably worth reorder the members so that
> the most distinguishing elements come first, to make it quicker to detect hash
> collisions. It shows up in profiles today...
>
> I guess it should be blockNum, fileNumber, forkNumber, dbOid, spcOid? I think
> as long as blockNum, fileNumber are first, the rest doesn't matter much.

Hmm, I guess we could do that. Possibly as a separate, very small patch.

> > And then like this in 0003:
> >
> > typedef struct buftag
> > {
> > Oid spcOid;
> > Oid dbOid;
> > RelFileNumber fileNumber:56;
> > ForkNumber forkNum:8;
> > } BufferTag;
>
> Probably worth checking the generated code / the performance effects of using
> bitfields (vs manual maskery). I've seen some awful cases, but here it's at a
> byte boundary, so it might be ok.

One advantage of using bitfields is that it might mean we don't need
to introduce accessor macros. Now, if that's going to lead to terrible
performance I guess we should go ahead and add the accessor macros -
Dilip had those in an earlier patch anyway. But it'd be nice if it
weren't necessary.

--
Robert Haas
EDB: http://www.enterprisedb.com