Re: Make relfile tombstone files conditional on WAL level

On Mon, Feb 7, 2022 at 11:31 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> For RelFileNode also we need to use 2, 32-bit integers so that we do
> not add extra alignment padding because there are a few more
> structures that include RelFileNode e.g. xl_xact_relfilenodes,
> RelFileNodeBackend, and many other structures.

Are you sure that kind of stuff is really important enough to justify
the code churn? I don't think RelFileNodeBackend is used widely enough
or in sufficiently performance-critical places that we really need to
care about a few bytes of alignment padding. xl_xact_relfilenodes is
more concerning because that goes into the WAL format, but I don't
know that we use it often enough for an extra 4 bytes per record to
really matter, especially considering that this proposal also adds 4
bytes *per relfilenode* which has to be a much bigger deal than a few
padding bytes after 'nrels'. The reason why BufferTag matters a lot is
because (1) we have an array of this struct that can easily contain a
million or eight entries, so the alignment padding adds up a lot more
and (2) access to that array is one of the most performance-critical
parts of PostgreSQL.

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