Re: Parallel Apply

On Tue, Aug 12, 2025 at 10:40 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>
> On Mon, Aug 11, 2025 at 10:15:41AM +0530, Amit Kapila wrote:
> > Hi,
> >
> > Background and Motivation
> > -------------------------------------
> > In high-throughput systems, where hundreds of sessions generate data
> > on the publisher, the subscriber's apply process often becomes a
> > bottleneck due to the single apply worker model. While users can
> > mitigate this by creating multiple publication-subscription pairs,
> > this approach has scalability and usability limitations.
> >
> > Currently, PostgreSQL supports parallel apply only for large streaming
> > transactions (streaming=parallel). This proposal aims to extend
> > parallelism to non-streaming transactions, thereby improving
> > replication performance in workloads dominated by smaller, frequent
> > transactions.
>
> I thought the approach for improving WAL apply speed, for both binary
> and logical, was pipelining:
>
> https://en.wikipedia.org/wiki/Instruction_pipelining
>
> rather than trying to do all the steps in parallel.
>

It is not clear to me how the speed for a mix of dependent and
independent transactions can be improved using the technique you
shared as we still need to follow the commit order for dependent
transactions. Can you please elaborate more on the high-level idea of
how this technique can be used to improve speed for applying logical
WAL records?

--
With Regards,
Amit Kapila.