Proposal: new GUC paramter

I see following in src/backend/optimizer/plan/planner.c

if (child && IsA(child, FromExpr))
{

/*
* Yes, so do we want to merge it into parent? Always do
* so if child has just one element (since that doesn't
* make the parent's list any longer). Otherwise we have
* to be careful about the increase in planning time
* caused by combining the two join search spaces into
* one. Our heuristic is to merge if the merge will
* produce a join list no longer than GEQO_RELS/2.
* (Perhaps need an additional user parameter?)
*/

This is really annoying since:

o these code fragments actually controls the optimization efforts for
subqueries and views, not related to GEQO at all. So using GEQO
parameters for this kind of purpose seems abuse for me.

o Even if geqo = false in postgresql.con, the code looks into the GEQO
value. This is really confusing for users.

So I propose a new GUC parameter called "subquery_merge_threshold"
solely for this purpose.

Comments?
--
Tatsuo Ishii