Re: pass-through queries to foreign servers - Mailing list pgsql-hackers

David Gudeman <dave.gudeman@gmail.com> writes:
> For those who don't want to go to the link to see what I'm talking
> about with query rewrites, I thought I'd give a brief description.
> Foreign data wrappers currently do all of their work in the planning
> phase but I claim that isn't the right place to optimize foreign
> queries with aggregates and GROUP BY because optimizing those things
> would involve collapsing multiple plan node back into a single node
> for a foreign call.

I'm not sure what the best implementation for that is, but what you
propose here would still involve such collapsing, so this argument
seems rather empty.

> I propose to do these optimizations as query
> rewrites instead. So for example suppose t is a foreign table on the
> foreign server named fs. Then the query

>   SELECT count(*) FROM t

> is rewritten to

>   SELECT count FROM fs('select count(*) from t') fs(count bigint)

> where ts() is the pass-through query function for the server fs. To
> implement this optimization as a query rewrite, all of the elements of
> the result have to be real source-language constructs so the
> pass-through query has to be available in Postgresql SQL.

I don't believe in any part of that design, starting with the "pass
through query function".  For one thing, it seems narrowly targeted to the
assumption that the FDW is a frontend for a foreign server that speaks
SQL.  If the FDW's infrastructure doesn't include some kind of textual
query language, this isn't going to be useful for it at all.  For another,
a query rewrite system is unlikely to be able to cost out the alternatives
and decide whether pushing the aggregation across is actually a win or
not.

The direction I think we ought to be heading is to generate explicit Paths
representing the various ways in which aggregation can be implemented.
The logic in grouping_planner is already overly complex, and hard to
extend, because it's all hard-wired comparisons of alternatives.  We'd be
better off with something more like the add_path infrastructure.  Once
that's been done, maybe we can allow FDWs to add Paths representing remote
aggregation.

                        regards, tom lane

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