Re: [Proposal] Fully WAL logged CREATE DATABASE - No Checkpoints

Hi,

On 2022-02-17 18:00:19 -0500, Robert Haas wrote:
> Now you pointed out earlier one problem that it doesn't fix: unlike
> the current method, this method involves reading buffers from the
> template database into shared_buffers, and those buffers, once read,
> stick around even after the operation finishes.

Yea, I don't see a problem with that. A concurrent DROP DATABASE or such would
be problematic, but the locking prevents that.

> The only other kind of hazard I can think of is: could it be unsafe to
> try to interpret the contents of a database to which no one else is
> connected at present due to any of the issues you mention? But what's
> the hazard exactly?

I don't quite know. But I don't think it's impossible to run into trouble in
this area. E.g. xid horizons are computed in a database specific way. If the
routine reading pg_class did hot pruning, you could end up removing data
that's actually needed for a logical slot in the other database because the
backend local horizon state was computed for the "local" database?

Could there be problems because other backends wouldn't see the backend
accessing the other database as being connected to that database
(PGPROC->databaseId)?

Or what if somebody optimized snapshots to disregard readonly transactions in
other databases?

> It can't be a problem if we've failed to process sinval messages for the
> target database, because we're not using any caches anyway.

Could you end up with an outdated relmap entry? Probably not.

> We can't. It can't be unsafe to test visibility of XIDs for that database,
> because in an alternate universe some backend could have connected to that
> database and seen the same XIDs.

That's a weak argument, because in that alternative universe a PGPROC entry
with the PGPROC->databaseId = template_databases_oid would exist.

Greetings,

Andres Freund