Re: Join push-down support for foreign tables

On Sun, Sep 7, 2014 at 7:07 PM, Shigeru HANADA <shigeru.hanada@gmail.com> wrote:
>> I think it's probably good to give an FDW the option of producing a
>> ForeignJoinPath for any join against a ForeignPath *or
>> ForeignJoinPath* for the same FDW.  It's perhaps unlikely that an FDW
>> can perform a join efficiently between two data sources with different
>> server definitions, but why not give it the option?  It should be
>> pretty fast for the FDW to realize, oh, the server OIDs don't match -
>> and at that point it can exit without doing anything further if that
>> seems desirable.  And there might be some kinds of data sources where
>> cross-server joins actually can be executed quickly (e.g. when the
>> underlying data is just in two files in different places on the local
>> machine).
>
> Indeed how to separate servers is left to users, or author of FDWs, though
> postgres_fdw and most of other FDWs can join foreign tables in a server.  I
> think it would be good if we can know two foreign tables are managed by same
> FDW, from FdwRoutine, maybe adding new API which returns FDW identifier?

Do we need this?  I mean, if you get the FdwRoutine, don't you have
the OID of the FDW or the foreign table also?

> I think so too, so ForeignJoinPath should be able to be an input of another
> ForeignJoinPath in upper join level.  But I also think joining on remote or
> not should be decided based on cost, as existing joins are planned with
> bottom-up approach.

Definitely.

-- 
Robert Haas
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