Re: Missing empty transaction optimization in pgoutput plugin

On Wed, Dec 10, 2025 at 9:17 AM Zhijie Hou (Fujitsu)
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> On Monday, December 8, 2025 2:49 PM Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> wrote:
> >
> > Hi All,
> > test_decoding output plugin skips sending stream_start and stream_end if no
> > change from a segment of a streamed transaction is sent downstream. If no
> > change has been sent downstream across all the streamed segments it
> > doesn't send commit or abort for that streamed transaction. But I don't see
> > similar changes in stream related callbacks in pgoutput output plugin.
> >
> > While the changes discussed in thread [1] were committed, those didn't
> > include this optimization for test_decoding plugin as well. The optimization
> > was discussed in [2] separately as a bug fix. Maybe because it wasn't
> > considered as an optimization that time, we may not have considered it for
> > pgoutput plugin which doesn't have explicit skip-empty-xacts option. I think
> > the optimization would be useful in setups where multiple publications and
> > long transactions are common.
> > Any reason not to have it.
>
> One possible reason we did not implement the optimization to avoid sending empty stream
> blocks along with commit d5a9d86 is the handling of prepared transactions, for
> which STREAM PREPARE is sent. The logical replication worker currently relies on
> receiving at least one STREAM START/STREAM STOP prior to this; otherwise, it
> might encounter errors when applying STREAM PREPARE. Although the optimization
> is feasible, it requires some modifications to ensure the replication worker works
> correctly, such as sending an empty stream block before issuing STREAM PREPARE.
>
> One might think that we could avoid sending STREAM PREPARE for empty txn as
> well, but that could be tricky, see the comment atop of (typedef struct
> PGOutputTxnData) for details.
>
> >
> > [1] https://www.postgresql.org/message-id/flat/688b0b7f-2f6c-d827-c27b-
> > 216a8e3ea700%402ndquadrant.com
> > [2] https://www.postgresql.org/message-
> > id/flat/CAA4eK1%2BOqgFNZkf7%3DETe_y5ntjgDk3T0wcdkd4Sot_u1hySGfw%
> > 40mail.gmail.com
>

Thanks a lot for the details. I see it's much larger change than just
a few lines like test_decoding.c.

--
Best Wishes,
Ashutosh Bapat