Re: [HACKERS] Proposal: Pluggable Optimizer Interface

Julius Stroffek <Julius(dot)Stroffek(at)Sun(dot)COM> writes:
> I understood that if the user creates his own implementation of the
> planner which can be stored in some external library, he have to provide
> some C language function as a "hook activator" which will assign the
> desired value to the planner_hook variable. Both, the activator function
> and the new planner implementation have to be located in the same
> dynamic library which will be loaded when CREATE FUNCTION statement
> would be used on "hook activator" function.

You could do it that way if you wanted, but a minimalistic solution is
just to install the hook from the _PG_init function of a loadable
library, and then LOAD is sufficient for a user to execute the thing.
There's a small example at
http://archives.postgresql.org/pgsql-patches/2007-05/msg00421.php

Also, having the loadable module add a custom GUC variable would likely
be a preferable solution for control purposes than making specialized
functions. I attach another small hack I made recently, which simply
scales all the planner's relation size estimates by a scale_factor GUC;
this is handy for investigating how a plan will change with relation
size, without having to actually create gigabytes of test data.

> There are more things in the proposal as a new pg_optimizer catalog and
> different way of configuring the hooks. However, this thinks are not
> mandatory for the functionality but are more user friendly.

Granted, but at this point we are talking about infrastructure for
planner-hackers to play with, not something that's intended to be a
long-term API for end users. It may or may not happen that we ever
need a user API for this at all. I think a planner that just "does the
right thing" is far preferable to one with a lot of knobs that users
have to know how to twiddle, so I see this more as scaffolding on which
someone can build and test the replacement for GEQO; which ultimately
would go in without any user-visible API additions.

regards, tom lane