Re: Database-level collation version tracking

On Tue, Feb 08, 2022 at 12:14:02PM +0100, Peter Eisentraut wrote:
> On 07.02.22 17:08, Julien Rouhaud wrote:
> > There's so limited testing in collate.* regression tests, so I thought it would
> > be ok to add it there. At least some ALTER DATABASE ... REFRESH VERSION would
> > be good, similarly to collation-level versioning.
>
> I don't think you can run ALTER DATABASE from the regression test scripts,
> since the database name is not fixed. You'd have to paste the command
> together using psql tricks or something. I guess it could be done, but
> maybe there is a better solution. We could put it into the createdb test
> suite, or write a new TAP test suite somewhere. There is no good precedent
> for this.

I was thinking using a simple DO command, as there are already some other usage
for that for non deterministic things (like TOAST tables). If it's too
problematic I'm fine with not having tests for that for now.

> > > That code takes a RowExclusiveLock on pg_database. Did you have something
> > > else in mind?
> >
> > But that lock won't prevent a concurrent DROP DATABASE, so it's totally
> > expected to hit that cache lookup failed error. There should either be a
> > shdepLockAndCheckObject(), or changing the error message to some errmsg("data
> > xxx does not exist").
>
> I was not familiar with that function. AFAICT, it is only used for database
> and role settings (AlterDatabaseSet(), AlterRoleSet()), but not when just
> updating the role or database catalog (e.g., AlterRole(), RenameRole(),
> RenameDatabase()). So I don't think it is needed here. Maybe I'm missing
> something.

I'm just saying that without such a lock you can easily trigger the "cache
lookup" error, and that's something that's supposed to happen with normal
usage I think. So it should be a better message saying that the database has
been concurrently dropped, or actually simply does not exist like it's done in
AlterDatabaseOwner() for the same pattern:

[...]
tuple = systable_getnext(scan);
if (!HeapTupleIsValid(tuple))
ereport(ERROR,
(errcode(ERRCODE_UNDEFINED_DATABASE),
errmsg("database \"%s\" does not exist", dbname)));
[...]

Apart from that I still think that we should check the collation version of the
source database when creating a new database. It won't cost much but will give
the DBA a chance to recreate the indexes before risking invalid index usage.