Re: ALTER TABLE .. DETACH PARTITION CONCURRENTLY

On Mon, Apr 12, 2021 at 6:23 AM Justin Pryzby <pryzby@telsasoft.com> wrote:
> On Sun, Apr 11, 2021 at 05:20:35PM -0400, Alvaro Herrera wrote:
> > On 2021-Mar-31, Tom Lane wrote:
> >
> > > diff -U3 /home/buildfarm/trilobite/buildroot/HEAD/pgsql.build/src/test/isolation/expected/detach-partition-concurrently-4.out /home/buildfarm/trilobite/buildroot/HEAD/pgsql.build/src/test/isolation/output_iso/results/detach-partition-concurrently-4.out
> > > --- /home/buildfarm/trilobite/buildroot/HEAD/pgsql.build/src/test/isolation/expected/detach-partition-concurrently-4.out    2021-03-29 20:14:21.258199311 +0200
> > > +++ /home/buildfarm/trilobite/buildroot/HEAD/pgsql.build/src/test/isolation/output_iso/results/detach-partition-concurrently-4.out  2021-03-30 18:54:34.272839428 +0200
> > > @@ -324,6 +324,7 @@
> > >  1
> > >  2
> > >  step s1insert: insert into d4_fk values (1);
> > > +ERROR:  insert or update on table "d4_fk" violates foreign key constraint "d4_fk_a_fkey"
> > >  step s1c: commit;
> > >
> > >  starting permutation: s2snitch s1b s1s s2detach s1cancel s3vacfreeze s1s s1insert s1c
> >
> > Hmm, actually, looking at this closely, I think the expected output is
> > bogus and trilobite is doing the right thing by throwing this error
> > here.  The real question is why isn't this case behaving in that way in
> > every *other* animal.
>
> I was looking/thinking at it, and wondered whether it could be a race condition
> involving pg_cancel_backend()

I thought about it some and couldn't come up with an explanation as to
why pg_cancel_backend() race might be a problem.

Actually it occurred to me this morning that CLOBBER_CACHE_ALWAYS is
what exposed this problem on this animal (not sure if other such
animals did too though).  With CLOBBER_CACHE_ALWAYS, a PartitionDesc
will be built afresh on most uses.  In this particular case, the RI
query executed by the insert has to build a new one (for d4_primary),
correctly excluding the detach-pending partition (d4_primary1) per the
snapshot with which it is run.  In normal builds, it would reuse the
one built by an earlier query in the transaction, which does include
the detach-pending partition, thus allowing the insert trying to
insert a row referencing that partition to succeed.  There is a
provision in RelationGetPartitionDesc() to rebuild if any
detach-pending partitions in the existing copy of PartitionDesc are
not to be seen by the current query, but a bug mentioned in my earlier
reply prevents that from kicking in.


--
Amit Langote
EDB: http://www.enterprisedb.com