Re: [PoC] Asynchronous execution again (which is not parallel)

On Wed, Dec 16, 2015 at 1:34 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> Yes, thats one thing I wanted to know, yet another point which is not
> clear to me about this Async infrastructure is why the current
> infrastructure
> of Parallelism can't be used to achieve the Async benefits of ForeignScan?

Well, all a ForeignScan by postgres_fdw does is read the tuples that
are generated remotely.  Turning around and sticking those into a
Funnel doesn't seem like it gains much: now instead of having to read
tuples from someplace, the leader has to read tuples from some other
place.  Yeah, there are cases where it could win, like when there's a
selective nonpushable qual, but that's not that exciting.

There's another, more serious problem: if the leader has a connection
open to the remote server and that connection is in mid-transaction,
you can't have a worker open a new connection without changing the
semantics.  Working around that problem looks hard to me.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company