Re: patch: SQL/MED(FDW) DDL

On Thu, Sep 30, 2010 at 10:29:56PM -0400, Robert Haas wrote:
> On Sep 29, 2010, at 10:09 AM, Alvaro Herrera <alvherre(at)commandprompt(dot)com> wrote:
> > Excerpts from Robert Haas's message of mar sep 28 10:26:54 -0400 2010:
> >
> >> Then:
> >>
> >> - Begin a sequential scan with the following set of quals.
> >> - Begin an index scan using the index called X with the following set of quals.
> >> - Fetch next tuple.
> >> - End scan.
> >
> > I'm not sure that it's a good idea to embed into the FDW API the
> > set of operations known to the executor. For example your
> > proposal fails to consider bitmap scans. Seems simpler and more
> > general to hand the quals over saying "I need to scan this
> > relation with these quals", and have it return an opaque iterable
> > object; the remote executor would be in charge of determining
> > their usage for indexscans; or even for filtering tuples with
> > quals that cannot be part of the index condition.
>
> Yeah, that might be better. Is it reasonable to assume we always
> want to push down as much as possible, or do we need to think about
> local work vs. remote work trade-offs?

In cases where the foreign side is not super reliable for correctness,
I'd say there's a good case for not trusting it. Some of the Oracle
properties are like this. I suppose we might want to make "trust the
other side to handle pushed quals" optional somehow, but I'm not
exactly sure how.

> > There doesn't to be much point in knowing the names of remote
> > indexes either (if it came to referencing them, better use OIDs)
>
> Well, you can't assume the remote side is PG.

Very definitely not.

The main point of the feature is that it allows for *heterogeneous*
data stores. Of course this doesn't prevent PostgreSQL from
optimizing aggressively using secret knowledge should the "foreign"
side happen to be PostgreSQL.

Cheers,
David.
--
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