Re: Asynchronous Append on postgres_fdw nodes.

On 6/10/20 8:05 AM, Kyotaro Horiguchi wrote:
> Hello, Andrey.
>
> At Tue, 9 Jun 2020 14:20:42 +0500, Andrey Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru> wrote in
>> On 6/4/20 11:00 AM, Kyotaro Horiguchi wrote:
>> 2. Total cost of an Append node is a sum of the subplans. Maybe in the
>> case of asynchronous append we need to use some reduce factor?
>
> Yes. For the reason mentioned above, foreign subpaths don't affect
> the startup cost of Append as far as any sync subpaths exist. If no
> sync subpaths exist, the Append's startup cost is the minimum startup
> cost among the async subpaths.
I mean that you can possibly change computation of total cost of the
Async append node. It may affect the planner choice between ForeignScan
(followed by the execution of the JOIN locally) and partitionwise join
strategies.

Have you also considered the possibility of dynamic choice between
synchronous and async append (during optimization)? This may be useful
for a query with the LIMIT clause.

--
Andrey Lepikhov
Postgres Professional