Re: Custom Scan APIs (Re: Custom Plan node)

>> Yeah.  I'm still not exactly convinced that custom-scan will ever allow
>> independent development of new plan types (which, with all due respect to
>> Robert, is what it was being sold as last year in Ottawa).  But I'm not
>> opposed in principle to committing it, if we can find a way to have a cleaner
>> API for things like setrefs.c.  It seems like late-stage planner processing
>> in general is an issue for this patch (createplan.c and subselect.c are
>> also looking messy).  EXPLAIN isn't too great either.
>>
>> I'm not sure exactly what to do about those cases, but I wonder whether
>> things would get better if we had the equivalent of
>> expression_tree_walker/mutator capability for plan nodes.  The state of
>> affairs in setrefs and subselect, at least, is a bit reminiscent of the
>> bad old days when we had lots of different bespoke code for traversing
>> expression trees.
>>
> Hmm. If we have something like expression_tree_walker/mutator for plan nodes,
> we can pass a walker/mutator function's pointer instead of exposing static
> functions that takes recursive jobs.
> If custom-plan provider (that has sub-plans) got a callback with walker/
> mutator pointer, all it has to do for sub-plans are calling this new
> plan-tree walking support routine with supplied walker/mutator.
> It seems to me more simple design than what I did.
>
I tried to code the similar walker/mutator functions on plan-node tree,
however, it was not available to implement these routines enough
simple, because the job of walker/mutator functions are not uniform
thus caller side also must have a large switch-case branches.

I picked up setrefs.c for my investigation.
The set_plan_refs() applies fix_scan_list() on the expression tree being
appeared in the plan node if it is delivered from Scan, however, it also
applies set_join_references() for subclass of Join, or
set_dummy_tlist_references() for some other plan nodes.
It implies that the walker/mutator functions of Plan node has to apply
different operation according to the type of Plan node. I'm not certain
how much different forms are needed.
(In addition, set_plan_refs() performs usually like a walker, but
often performs as a mutator if trivial subquery....)

I'm expecting the function like below. It allows to call plan_walker
function for each plan-node and also allows to call expr_walker
function for each expression-node on the plan node.
   bool   plan_tree_walker(Plan *plan,                    bool (*plan_walker) (),                    bool
(*expr_walker)(),                    void *context)
 

I'd like to see if something other form to implement this routine.


One alternative idea to give custom-plan provider a chance to
handle its subplans is, to give function pointers (1) to handle
recursion of plan-tree and (2) to set up backend's internal
state.
In case of setrefs.c, set_plan_refs() and fix_expr_common()
are minimum necessity for extensions. It also kills necessity
to export static functions.

How about your thought?
-- 
KaiGai Kohei <kaigai@kaigai.gr.jp>