diff --git a/src/backend/executor/nodeHash.c b/src/backend/executor/nodeHash.c
index 906cb46b65..ee9298a157 100644
--- a/src/backend/executor/nodeHash.c
+++ b/src/backend/executor/nodeHash.c
@@ -542,6 +542,9 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
 			nbatch <<= 1;
 	}
 
+	Assert(nbuckets > 0);
+	Assert(nbatch > 0);
+
 	*numbuckets = nbuckets;
 	*numbatches = nbatch;
 }
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 6ee411eec8..09d4ea12e8 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -1536,9 +1536,11 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 								  standard_qp_callback, &qp_extra);
 
 		/*
-		 * Extract rowcount and width estimates for use below.
+		 * Extract rowcount and width estimates for use below.  If final_rel
+		 * has been proven dummy, its rows estimate will be zero; clamp it to
+		 * one to avoid zero-divide in subsequent calculations.
 		 */
-		path_rows = final_rel->rows;
+		path_rows = clamp_row_est(final_rel->rows);
 		path_width = final_rel->width;
 
 		/*
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 64b6ae4838..14b8c2ff54 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -4632,8 +4632,8 @@ examine_simple_variable(PlannerInfo *root, Var *var,
  * *isdefault: set to TRUE if the result is a default rather than based on
  * anything meaningful.
  *
- * NB: be careful to produce an integral result, since callers may compare
- * the result to exact integer counts.
+ * NB: be careful to produce a positive integral result, since callers may
+ * compare the result to exact integer counts, or might divide by it.
  */
 double
 get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
@@ -4709,7 +4709,7 @@ get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
 	 * If we had an absolute estimate, use that.
 	 */
 	if (stadistinct > 0.0)
-		return stadistinct;
+		return clamp_row_est(stadistinct);
 
 	/*
 	 * Otherwise we need to get the relation size; punt if not available.
@@ -4730,7 +4730,7 @@ get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
 	 * If we had a relative estimate, use that.
 	 */
 	if (stadistinct < 0.0)
-		return floor((-stadistinct * ntuples) + 0.5);
+		return clamp_row_est(-stadistinct * ntuples);
 
 	/*
 	 * With no data, estimate ndistinct = ntuples if the table is small, else
@@ -4738,7 +4738,7 @@ get_variable_numdistinct(VariableStatData *vardata, bool *isdefault)
 	 * that the behavior isn't discontinuous.
 	 */
 	if (ntuples < DEFAULT_NUM_DISTINCT)
-		return ntuples;
+		return clamp_row_est(ntuples);
 
 	*isdefault = true;
 	return DEFAULT_NUM_DISTINCT;