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

On Mon, Nov 16, 2020 at 3:20 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Mon, Nov 16, 2020 at 4:25 PM Ajin Cherian <itsajin(at)gmail(dot)com> wrote:
> >
> > Updated with a new test case
> > (contrib/test_decoding/t/002_twophase-streaming.pl) that tests
> > concurrent aborts during streaming prepare. Had to make a few changes
> > to the test_decoding stream_start callbacks to handle
> > "check-xid-aborted"
> > the same way it was handled in the non stream callbacks. Merged
> > Peter's v20-0006 as well.
> >
>
> Thank you for updating the patch.
>
> I have a question about the timestamp of PREPARE on a subscriber node,
> although this may have already been discussed.
>
> With the current patch, the timestamps of PREPARE are different
> between the publisher and the subscriber but the timestamp of their
> commits are the same. For example,
>
> -- There is 1 prepared transaction on a publisher node.
> =# select * from pg_prepared_xact;
>
> transaction | gid | prepared | owner | database
> -------------+-----+-------------------------------+----------+----------
> 510 | h1 | 2020-11-16 16:57:13.438633+09 | masahiko | postgres
> (1 row)
>
> -- This prepared transaction is replicated to a subscriber.
> =# select * from pg_prepared_xact;
>
> transaction | gid | prepared | owner | database
> -------------+-----+-------------------------------+----------+----------
> 514 | h1 | 2020-11-16 16:57:13.440593+09 | masahiko | postgres
> (1 row)
>
> These timestamps are different. Let's commit the prepared transaction
> 'h1' on the publisher and check the commit timestamps on both nodes.
>
> -- On the publisher node.
> =# select pg_xact_commit_timestamp('510'::xid);
>
> pg_xact_commit_timestamp
> -------------------------------
> 2020-11-16 16:57:13.474275+09
> (1 row)
>
> -- Commit prepared is also replicated to the subscriber node.
> =# select pg_xact_commit_timestamp('514'::xid);
>
> pg_xact_commit_timestamp
> -------------------------------
> 2020-11-16 16:57:13.474275+09
> (1 row)
>
> The commit timestamps are the same. At PREPARE we use the local
> timestamp when PREPARE is executed as 'prepared' time while at COMMIT
> PREPARED we use the origin's commit timestamp as the commit timestamp
> if the commit WAL has.
>

Doesn't this happen only if you set replication origins? Because
otherwise both PrepareTransaction() and
RecordTransactionCommitPrepared() used the current timestamp.

--
With Regards,
Amit Kapila.