Re: Join push-down support for foreign tables

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.

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

+ Everyone has their own god. +