RE: Synchronizing slots from primary to standby

On Tuesday, April 2, 2024 8:43 AM Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com> wrote:
> 
> On Mon, Apr 1, 2024 at 11:36 AM Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>
> wrote:
> >
> > Attach the V4 patch which includes the optimization to skip the
> > decoding if the snapshot at the syncing restart_lsn is already
> > serialized. It can avoid most of the duplicate decoding in my test, and I am
> doing some more tests locally.
> 
> Thanks for the patch. I'm thinking if we can reduce the amount of work that we
> do for synced slots in each sync worker cycle. With that context in mind, why do
> we need to create decoding context every time?
> Can't we create it once, store it in an in-memory structure and use it for each
> sync worker cycle? Is there any problem with it? What do you think?

Thanks for the idea. I think the cost of decoding context seems to be
relatively minor when compared to the IO cost. After generating the profiles
for the tests shared by Nisha[1], it appears that the StartupDecodingContext is
not a issue. While the suggested refactoring is an option, I think
we can consider this as a future improvement and addressing it only if we
encounter scenarios where the creation of decoding context becomes a
bottleneck.

[1] https://www.postgresql.org/message-id/CALj2ACUeij5tFzJ1-cuoUh%2Bmhj33v%2BYgqD_gHYUpRdXSCSBbhw%40mail.gmail.com

Best Regards,
Hou zj