Invent recursive_worktable_factor GUC to replace hard-wired constant.

Up to now, the planner estimated the size of a recursive query's
worktable as 10 times the size of the non-recursive term.  It's hard
to see how to do significantly better than that automatically, but
we can give users control over the multiplier to allow tuning for
specific use-cases.  The default behavior remains the same.

Simon Riggs

Discussion: https://postgr.es/m/CANbhV-EuaLm4H3g0+BSTYHEGxJj3Kht0R+rJ8vT57Dejnh=_nA@mail.gmail.com

src/backend/utils/misc/postgresql.conf.sample

@@ -426,6 +426,7 @@
 					# JOIN clauses
 #plan_cache_mode = auto			# auto, force_generic_plan or
 					# force_custom_plan
+#recursive_worktable_factor = 10.0	# range 0.001-1000000
 
 
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