Adjust estimate_num_groups() to not clamp per-relation group count

estimate to less than the number of values estimated for any one grouping
Var, as suggested by Manfred.  This is intuitively right, and what's
more it puts the plan choices in the subselect regression test back the
way they were before ...

src/backend/utils/adt/selfuncs.c

@@ -15,7 +15,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.170 2005/01/28 20:34:25 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.171 2005/02/01 23:07:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2043,6 +2043,7 @@ estimate_num_groups(Query *root, List *groupExprs, double input_rows)
 		GroupVarInfo *varinfo1 = (GroupVarInfo *) linitial(varinfos);
 		RelOptInfo *rel = varinfo1->rel;
 		double		reldistinct = varinfo1->ndistinct;
+		double		relmaxndistinct = reldistinct;
 		int			relvarcount = 1;
 		List	   *newvarinfos = NIL;
 
@@ -2057,6 +2058,8 @@ estimate_num_groups(Query *root, List *groupExprs, double input_rows)
 			if (varinfo2->rel == varinfo1->rel)
 			{
 				reldistinct *= varinfo2->ndistinct;
+				if (relmaxndistinct < varinfo2->ndistinct)
+					relmaxndistinct = varinfo2->ndistinct;
 				relvarcount++;
 			}
 			else
@@ -2075,12 +2078,23 @@ estimate_num_groups(Query *root, List *groupExprs, double input_rows)
 			/*
 			 * Clamp to size of rel, or size of rel / 10 if multiple Vars.
 			 * The fudge factor is because the Vars are probably correlated
-			 * but we don't know by how much.
+			 * but we don't know by how much.  We should never clamp to less
+			 * than the largest ndistinct value for any of the Vars, though,
+			 * since there will surely be at least that many groups.
 			 */
 			double		clamp = rel->tuples;
 
 			if (relvarcount > 1)
+			{
 				clamp *= 0.1;
+				if (clamp < relmaxndistinct)
+				{
+					clamp = relmaxndistinct;
+					/* for sanity in case some ndistinct is too large: */
+					if (clamp > rel->tuples)
+						clamp = rel->tuples;
+				}
+			}
 			if (reldistinct > clamp)
 				reldistinct = clamp;
 

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
-      |                3
-      |                2
       |                2
+      |                3
 (5 rows)
 
 --