Re: Join push-down support for foreign tables

On Thu, Sep 4, 2014 at 08:41:43PM +0530, Atri Sharma wrote:
>
>
> On Thursday, September 4, 2014, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>
> On Thu, Sep  4, 2014 at 08:37:08AM -0400, Robert Haas wrote:
> > The main problem I see here is that accurate costing may require a
> > round-trip to the remote server.  If there is only one path that is
> > probably OK; the cost of asking the question will usually be more than
> > paid for by hearing that the pushed-down join clobbers the other
> > possible methods of executing the query.  But if there are many paths,
> > for example because there are multiple sets of useful pathkeys, it
> > might start to get a bit expensive.
> >
> > Probably both the initial cost and final cost calculations should be
> > delegated to the FDW, but maybe within postgres_fdw, the initial cost
> > should do only the work that can be done without contacting the remote
> > server; then, let the final cost step do that if appropriate.  But I'm
> > not entirely sure what is best here.
>
> I am thinking eventually we will need to cache the foreign server
> statistics on the local server.
>
>
>
>
> Wouldn't that lead to issues where the statistics get outdated and we have to
> anyways query the foreign server before planning any joins? Or are you thinking
> of dropping the foreign table statistics once the foreign join is complete?

I am thinking we would eventually have to cache the statistics, then get
some kind of invalidation message from the foreign server. I am also
thinking that cache would have to be global across all backends, I guess
similar to our invalidation cache.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +