Re: [HACKERS] logical decoding of two-phase transactions

On Wed, Oct 21, 2020 at 7:42 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Wed, Oct 21, 2020 at 1:38 PM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
> >
> > The PG docs for PREPARE TRANSACTION [1] don't say anything about an
> > empty (zero length) transaction-id.
> > e.g. PREPARE TRANSACTION '';
> > [1] https://www.postgresql.org/docs/current/sql-prepare-transaction.html
> >
> > ~
> >
> > Meanwhile, during testing I found the 2PC prepare hangs when an empty
> > id is used.
> >
>
> Can you please take an example to explain what you are trying to say?

I was referring to an empty (zero length) transaction ID, not an empty
transaction.

The example was already given as PREPARE TRANSACTION '';

A longer example from my regress test is shown below. Using 2PC
pub/sub this will currently hang:

# --------------------
# Test using empty GID
# --------------------
# check that 2PC gets replicated to subscriber
$node_publisher->safe_psql('postgres',
"BEGIN;INSERT INTO tab_full VALUES (51);PREPARE TRANSACTION '';");
$node_publisher->poll_query_until('postgres', $caughtup_query)
or die "Timed out while waiting for subscriber to catch up";
# check that transaction is in prepared state on subscriber
$result =
$node_subscriber->safe_psql('postgres', "SELECT count(*) FROM
pg_prepared_xacts where gid = '';");
is($result, qq(1), 'transaction is prepared on subscriber');
# ROLLBACK
$node_publisher->safe_psql('postgres',
"ROLLBACK PREPARED '';");
# check that 2PC gets aborted on subscriber
$node_publisher->poll_query_until('postgres', $caughtup_query)
or die "Timed out while waiting for subscriber to catch up";
$result =
$node_subscriber->safe_psql('postgres', "SELECT count(*) FROM
pg_prepared_xacts where gid = '';");
is($result, qq(0), 'transaction is aborted on subscriber');

~

Is that something that should be made to work for 2PC pub/sub, or was
Postgres PREPARE TRANSACTION statement wrong to allow the user to
specify an empty transaction ID in the first place?

Kind Regards
Peter Smith.
Fujitsu Australia.