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 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.
My current implementation of this uses a plugin that hooks into
planner_hook, but I'm hoping that I can get some support for adding
the query rewriting as callback functions for the FDW system.
Regards,
David Gudeman
http://unobtainabol.blogspot.com
