On Tue, 2020-04-07 at 00:53 -0400, Tom Lane wrote: > "David G. Johnston" <david.g.johnston@gmail.com> writes: > > On Monday, April 6, 2020, AC Gomez <antklc@gmail.com> wrote: > > > If you issue an ALTER SERVER command and there are active connections > > > with that server in use or new ones are coming in, what happens? Docs on > > > this command say nothing regarding active processing using the server > > > context and changes to it. So I assume it's just handled. > > Not sure if there are exceptions but assume that nothing external will > > change your current active session’s settings out from underneath you. > > Couple of comments here: > > * ALTER SERVER changes nothing until "pg_ctl reload" or similar is > issued.
I think you both mixed up ALTER SYSTEM and ALTER SERVER.
The details of when exactly an ALTER SERVER will affect a query that uses a foreign table on the server will vary depending on the implementation of the foreign data wrapper, but typically the settings that were in effect when the query was *planned* will be the ones used.
Typically, queries are planned right before they are executed. Any query that is currently executing will continue to do so, but queries planned after the ALTER SERVER will use the new values.
Sometimes plans are cached, but all cached plans that involve the changed server will be invalidated and re-planned after the ALTER SERVER.