Re: Prevent concurrent DROP SCHEMA when certain objects are being initially created in the namespace

Hi,

On 2018-09-05 01:05:54 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > On September 4, 2018 9:11:25 PM PDT, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> I think that line of thought leads to an enormous increase in locking
> >> overhead, for which we'd get little if any gain in usability. So my
> >> inclination is to make an engineering judgment that we won't fix this.
>
> > Haven't we already significantly started down this road, to avoid a lot of the "tuple concurrently updated" type errors?
>
> Not that I'm aware of. We do not take locks on schemas, nor functions,
> nor any other of the object types I mentioned.

Well, we kinda do, during some of their own DDL. CF
AcquireDeletionLock(), RangeVarGetAndCheckCreationNamespace(), and other
LockDatabaseObject() callers. The
RangeVarGetAndCheckCreationNamespace() even locks the schema an object
is created in , which is pretty much what we're discussing here.

I thinkt he problem with the current logic is more that the
findDependentObjects() scan doesn't use a "dirty" scan, so it doesn't
ever get to seeing conflicting operations.

> > Would expanding this a git further really be that noticeable?
>
> Frankly, I think it would be not so much "noticeable" as "disastrous".
>
> Making the overhead tolerable would require very large compromises
> in coverage, perhaps like "we'll only lock during DDL not DML".
> At which point I'd question why bother. We've seen no field reports
> (that I can recall offhand, anyway) that trace to not locking these
> objects.

Why would "we'll only lock during DDL not DML" be such a large
compromise? To me that's a pretty darn reasonable subset - preventing
corruption of the catalog contents is, uh, good?

Greetings,

Andres Freund