When dealing with multiple grouping columns coming from the same table,

clamp the estimated number of groups to table row count over 10, instead
of table row count; this reflects a heuristic that people probably won't
group over a near-unique set of columns, and the knowledge that we don't
currently have any way to estimate the correlation of the columns better
than guessing.  This change creates a trivial plan change in one of the
regression tests.

src/test/regress/expected/subselect.out

@@ -134,11 +134,11 @@ SELECT '' AS five, f1 AS "Correlated Field"
                      WHERE f3 IS NOT NULL);
  five | Correlated Field 
 ------+------------------
-      |                2
       |                3
       |                1
-      |                2
       |                3
+      |                2
+      |                2
 (5 rows)
 
 --