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

On Wed, Nov 18, 2020 at 7:54 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Tue, Nov 17, 2020 at 9:05 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Tue, Nov 17, 2020 at 5:02 PM Ajin Cherian <itsajin(at)gmail(dot)com> wrote:
> > >
> > > On Tue, Nov 17, 2020 at 10:14 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > >
> > > > Doesn't this happen only if you set replication origins? Because
> > > > otherwise both PrepareTransaction() and
> > > > RecordTransactionCommitPrepared() used the current timestamp.
> > > >
> > >
> > > I was also checking this, even if you set replicating origins, the
> > > preparedTransaction will reflect the local prepare time in
> > > pg_prepared_xacts. pg_prepared_xacts fetches this information
> > > from GlobalTransaction data which does not store the origin_timestamp;
> > > it only stores the prepared_at which is the local timestamp.
> > >
> >
> > Sure, but my question was does this difference in behavior happens
> > without replication origins in any way? The reason is that if it
> > occurs only with replication origins, I don't think we need to bother
> > about the same because that feature is not properly implemented and
> > not used as-is. See the discussion [1] [2]. OTOH, if this behavior can
> > happen without replication origins then we might want to consider
> > changing it.
>
> Logical replication workers always have replication origins, right? Is
> that what you meant 'with replication origins'?
>

I was thinking with respect to the publisher-side but you are right
that logical apply workers always have replication origins so the
effect will be visible but I think the same should be true on
publisher without this patch as well. Say, the user has set up
replication origin via pg_replication_origin_xact_setup and provided a
value of timestamp then also the same behavior will be there.

> IIUC logical replication workers always set the origin's commit
> timestamp as the commit timestamp of the replicated transaction. OTOH,
> the timestamp of PREPARE, ‘prepare’ of pg_prepared_xacts, always uses
> the local timestamp even if the caller of PrepareTransaction() sets
> replorigin_session_origin_timestamp. In terms of user-visible
> timestamps of transaction operations, I think users might expect these
> timestamps are matched between the origin and its subscribers. But the
> pg_xact_commit_timestamp() is a function of the commit timestamp
> feature whereas ‘prepare’ is a pure timestamp when the transaction is
> prepared. So I’m not sure these timestamps really need to be matched,
> though.
>

Yeah, I am not sure if it is a good idea for users to rely on this
especially if the same behavior is visible on the publisher as well.
We might want to think separately if there is a value in making
prepare-time to also rely on replorigin_session_origin_timestamp and
if so, that can be done as a separate patch. What do you think?

--
With Regards,
Amit Kapila.