diff --git a/dbms/include/DB/AggregateFunctions/AggregateFunctionAvg.h b/dbms/include/DB/AggregateFunctions/AggregateFunctionAvg.h
index 29756e27b29..e939f50d724 100644
--- a/dbms/include/DB/AggregateFunctions/AggregateFunctionAvg.h
+++ b/dbms/include/DB/AggregateFunctions/AggregateFunctionAvg.h
@@ -79,6 +79,7 @@ public:
 
 /** Принимает два аргумента - значение и условие.
   * Вычисляет среднее значений при выполнении условия.
+  * avgIf(x, cond) эквивалентно sum(cond ? x : 0) / sum(cond).
   */
 template <typename T>
 class AggregateFunctionAvgIf : public IAggregateFunctionHelper<AggregateFunctionAvgData>
diff --git a/dbms/include/DB/AggregateFunctions/AggregateFunctionSum.h b/dbms/include/DB/AggregateFunctions/AggregateFunctionSum.h
index 843aa876bb7..31b4262a306 100644
--- a/dbms/include/DB/AggregateFunctions/AggregateFunctionSum.h
+++ b/dbms/include/DB/AggregateFunctions/AggregateFunctionSum.h
@@ -94,7 +94,7 @@ public:
 };
 
 
-/// Считает сумму чисел при выполнении условия.
+/// Считает сумму чисел при выполнении условия. sumIf(x, cond) эквивалентно sum(cond ? x : 0).
 template <typename T>
 class AggregateFunctionSumIf : public IAggregateFunctionHelper<AggregateFunctionSumData<T> >
 {