Re: ALTER TABLE .. DETACH PARTITION CONCURRENTLY

On Mon, Apr 12, 2021 at 6:20 AM Alvaro Herrera <alvherre@alvh.no-ip.org> 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.

Indeed.

I can see a wrong behavior of RelationGetPartitionDesc() in a case
that resembles the above test case.

drop table if exists d4_primary, d4_primary1, d4_fk, d4_pid;
create table d4_primary (a int primary key) partition by list (a);
create table d4_primary1 partition of d4_primary for values in (1);
create table d4_primary2 partition of d4_primary for values in (2);
insert into d4_primary values (1);
insert into d4_primary values (2);
create table d4_fk (a int references d4_primary);
insert into d4_fk values (2);
create table d4_pid (pid int);

Session 1:
begin isolation level serializable;
select * from d4_primary;
 a
---
 1
 2
(2 rows)

Session 2:
alter table d4_primary detach partition d4_primary1 concurrently;
<waits>

Session 1:
-- can see d4_primary1 as detached at this point, though still scans!
select * from d4_primary;
 a
---
 1
 2
(2 rows)
insert into d4_fk values (1);
INSERT 0 1
end;

Session 2:
<alter-finishes>
ALTER TABLE

Session 1:
-- FK violated
select * from d4_primary;
 a
---
 2
(1 row)
select * from d4_fk;
 a
---
 1
(1 row)

The 2nd select that session 1 performs adds d4_primary1, whose detach
it *sees* is pending, to the PartitionDesc, but without setting its
includes_detached.  The subsequent insert's RI query, because it uses
that PartitionDesc as-is, returns 1 as being present in d4_primary,
leading to the insert succeeding.  When session 1's transaction
commits, the waiting ALTER proceeds with committing the 2nd part of
the DETACH, without having a chance again to check if the DETACH would
lead to the foreign key of d4_fk being violated.

It seems problematic to me that the logic of setting includes_detached
is oblivious of the special check in find_inheritance_children() to
decide whether "force"-include a detach-pending child based on
cross-checking its pg_inherit tuple's xmin against the active
snapshot.  Maybe fixing that would help, although I haven't tried that
myself yet.

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