I wonder how hard it would be to give Postgres a pluggable query planner, so that extensions can try to improve on its results. Then people could try things out

I wonder how hard it would be to give Postgres a pluggable query planner, so that extensions can try to improve on its results. Then people could try things out without Postgres committing to something too soon. I'm still learning what Postgres extensions are capable of. I've written a few but only to define functions & types. It seems like I've seen more "interesting" achievements though, like CitusDB seems able to rewrite queries. Can an extension even add syntax? EDIT: This seems to be the important part: https://github.com/citusdata/citus/blob/master/src/backend/distributed/shared_library_init.c https://github.com/citusdata/citus/blob/master/src/backend/d... I wonder what `planner_hook` is about. . . .