Simple constraint exclusion. For now, only child tables of inheritance

scans are candidates for exclusion; this should be fixed eventually.
Simon Riggs, with some help from Tom Lane.

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

@@ -173,6 +173,7 @@
 # - Other Planner Options -
 
 #default_statistics_target = 10	# range 1-1000
+#enable_constraint_exclusion = off
 #from_collapse_limit = 8
 #join_collapse_limit = 8	# 1 disables collapsing of explicit JOINs