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

On Fri, Sep 2, 2022 at 5:25 PM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
>
> On Tue, Aug 02, 2022 at 12:50:43PM -0500, Justin Pryzby wrote:
> > Also, if I understand correctly, this patch seems to assume that nobody is
> > connected to the source database. But what's actually enforced is just that
> > nobody *else* is connected. Is it any issue that the current DB can be used as
> > a source? Anyway, both of the above problems are reproducible using a
> > different database.
> >
> > |postgres=# CREATE DATABASE new TEMPLATE postgres STRATEGY wal_log;
> > |CREATE DATABASE
>
> On Thu, Aug 04, 2022 at 05:16:04PM -0500, Justin Pryzby wrote:
> > On Thu, Aug 04, 2022 at 06:02:50PM -0400, Tom Lane wrote:
> > > The "invalidation" comment bothered me for awhile, but I think it's
> > > fine: we know that no other backend can connect to the source DB
> > > because we have it locked,
> >
> > About that - is it any problem that the currently-connected db can be used as a
> > template? It's no issue for 2-phase commit, because "create database" cannot
> > run in an txn.
>
> Would anybody want to comment on this ?
> Is it okay that the *current* DB can be used as a template ?

I don't think there should be any problem with that. The main problem
could have been that since we are reading the pg_class tuple block by
block there could be an issue if someone concurrently modifies the
pg_class or there are some tuples that are inserted by the prepared
transaction. But in this case, the same backend can not have an open
prepared transaction while creating a database and that backend of
course can not perform any parallel operation as well.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com