diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 46c95b0..82a7f7f 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -3438,9 +3438,9 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 				reldistinct = clamp;
 
 			/*
-			 * Multiply by restriction selectivity.
+			 * Estimate number of distinct values expected in given number of rows.
 			 */
-			reldistinct *= rel->rows / rel->tuples;
+			reldistinct *= (1 - powl((reldistinct - 1) / reldistinct, rel->rows));
 
 			/*
 			 * Update estimate of total distinct groups.