Re: Online checksums patch - once again

On Wed, Jan 22, 2020 at 3:28 PM Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> > I think the argument about adding catalog flags adding overhead is
> > pretty much bogus. Fixed-width fields in catalogs are pretty cheap.
>
> If that's the general view, then yeah our "cost calculations" were
> off. I guess I may have been colored by the cost of adding statistics
> counters, and had that influence the thinking. Incorrect judgement on
> that cost certainly contributed to the decision. back then.

For either statistics or for pg_class, the amount of data that we have
to manage is proportional to the number of relations (which could be
big) multiplied by the data stored for each relation. But the
difference is that the stats file has to be rewritten, at least on a
per-database basis, very frequently, while pg_class goes through
shared-buffers and so doesn't provoke the same stupid
write-the-whole-darn-thing behavior. That is a pretty key difference,
IMHO.

Now, it would be nice to fix the stats system, but until we do, here we are.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company