Thread

Re: Logical replication prefetch

Konstantin Knizhnik <knizhnik@garret.ru> — 2025-07-13T12:36:30Z

On 13/07/2025 1:28 pm, Amit Kapila wrote:
> On Tue, Jul 8, 2025 at 12:06 PM Konstantin Knizhnik <knizhnik@garret.ru> wrote:
>> There is well known Postgres problem that logical replication subscriber
>> can not caught-up with publisher just because LR changes are applied by
>> single worker and at publisher changes are made by
>> multiple concurrent backends.
>>
> BTW, do you know how users deal with this lag? For example, one can
> imagine creating multiple pub-sub pairs for different sets of tables
> so that the workload on the subscriber could also be shared by
> multiple apply workers. I can also think of splitting the workload
> among multiple pub-sub pairs by using row filters


Yes, I saw that users starts several subscriptions/publications to 
receive and apply changes in parallel.
But it can not be considered as universal solution:
1. Not always there are multiple tables (or partitions of one one table) 
so that it it possible to split them between multiple publications.
2. It violates transactional behavior (consistency): if transactions 
update several tables included in different publications then applying 
this changes independently, we can observe at replica behaviour when one 
table is update - and another - not. The same is true for row filters.
3. Each walsender will have to scan WAL, so having N subscriptions we 
have to read and decode WAL N times.