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

On Thu, Oct 22, 2020 at 4:58 AM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
>
> 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.
>

oh, I got it confused with the system generated 32-bit TransactionId.
But now, I got what you were referring to.

> The example was already given as PREPARE TRANSACTION '';
>
>
> 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?
>

I don't see any problem with the empty transaction identifier used in
Prepare Transaction. This is just used as an identifier to uniquely
identify the transaction. If you try to use an empty string ('') more
than once for Prepare Transaction, it will give an error like below:
postgres=*# prepare transaction '';
ERROR: transaction identifier "" is already in use

So, I think this should work for pub/sub as well. Did you find out the
reason of hang?

--
With Regards,
Amit Kapila.