Short-circuit sort_inner_and_outer if there are no mergejoin clauses

In sort_inner_and_outer, we create mergejoin join paths by explicitly
sorting both relations on each possible ordering of the available
mergejoin clauses.  However, if there are no available mergejoin
clauses, we can skip this process entirely.

This patch introduces a check for mergeclause_list at the beginning of
sort_inner_and_outer and exits the function if it is found to be
empty.  This might help skip all the statements that come before the
call to select_outer_pathkeys_for_merge, including the build of
UniquePaths in the case of JOIN_UNIQUE_OUTER or JOIN_UNIQUE_INNER.

I doubt there's any measurable performance improvement, but throughout
the run of the regression tests, sort_inner_and_outer is called a
total of 44,424 times.  Among these calls, there are 11,064 instances
where mergeclause_list is found to be empty, which accounts for
approximately one-fourth.  I think this suggests that implementing
this shortcut is worthwhile.

Author: Richard Guo
Reviewed-by: Ashutosh Bapat
Discussion: https://postgr.es/m/CAMbWs48RKiZGFEd5A0JtztRY5ZdvVvNiHh0AKeuoz21F+0dVjQ@mail.gmail.com

src/backend/optimizer/path/joinpath.c

@@ -1382,6 +1382,10 @@ sort_inner_and_outer(PlannerInfo *root,
 	List	   *all_pathkeys;
 	ListCell   *l;
 
+	/* Nothing to do if there are no available mergejoin clauses */
+	if (extra->mergeclause_list == NIL)
+		return;
+
 	/*
 	 * We only consider the cheapest-total-cost input paths, since we are
 	 * assuming here that a sort is required.  We will consider