Function allow_star_schema_join

src/backend/optimizer/path/joinpath.c:465–476

* We override the param_source_rels heuristic to accept nestloop paths in * which the outer rel satisfies some but not all of the inner path's * parameterization. This is necessary to get good plans for star-schema * scenarios, in which a parameterized path for a large table may require * parameters from multiple small tables that will not get joined directly to * each other. We can handle

 * should be overridden, ie, it's okay to perform this join.
 */
static inline bool
allow_star_schema_join(PlannerInfo *root,
					   Relids outerrelids,
					   Relids inner_paramrels)
{
	/*
	 * It's a star-schema case if the outer rel provides some but not all of
	 * the inner rel's parameterization.
	 */
	return (bms_overlap(inner_paramrels, outerrelids) &&
			bms_nonempty_difference(inner_paramrels, outerrelids));
}

/*
 * paraminfo_get_equal_hashops