On Mon, 22 Jun 2020 at 14:56, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com> wrote: > > Hi, > > When a query on foreign table is executed from a local session using > postgres_fdw, as expected the local postgres backend opens a > connection which causes a remote session/backend to be opened on the > remote postgres server for query execution. > > One observation is that, even after the query is finished, the remote > session/backend still persists on the remote postgres server. Upon > researching, I found that there is a concept of Connection Caching for > the remote connections made using postgres_fdw. Local backend/session > can cache up to 8 different connections per backend. This caching is > useful as it avoids the cost of reestablishing new connections per > foreign query. > > However, at times, there may be situations where the long lasting > local sessions may execute very few foreign queries and remaining all > are local queries, in this scenario, the remote sessions opened by the > local sessions/backends may not be useful as they remain idle and eat > up the remote server connections capacity. This problem gets even > worse(though this use case is a bit imaginary) if all of > max_connections(default 100 and each backend caching 8 remote > connections) local sessions open remote sessions and they are cached > in the local backend. > > I propose to have a new session level GUC called > "enable_connectioncache"(name can be changed if it doesn't correctly > mean the purpose) with the default value being true which means that > all the remote connections are cached. If set to false, the > connections are not cached and so are remote sessions closed by the local backend/session at > the end of each remote transaction.
I've not looked at your patch deeply but if this problem is talking only about postgres_fdw I think we should improve postgres_fdw, not adding a GUC to the core. It’s not that all FDW plugins use connection cache and postgres_fdw’s connection cache is implemented within postgres_fdw, I think we should focus on improving postgres_fdw. I also think it’s not a good design that the core manages connections to remote servers connected via FDW. I wonder if we can add a postgres_fdw option for this purpose, say keep_connection [on|off]. That way, we can set it per server so that remote connections to the particular server don’t remain idle.
+1
I have not looked at the implementation, but I agree that here problem
is with postgres_fdw so we should try to solve that by keeping it limited
to postgres_fdw. I liked the idea of passing it as an option to the FDW
