diff --git a/src/AggregateFunctions/AggregateFunctionIntervalLengthSum.h b/src/AggregateFunctions/AggregateFunctionIntervalLengthSum.h
index 5a82d9e6a5c..93e81be93fc 100644
--- a/src/AggregateFunctions/AggregateFunctionIntervalLengthSum.h
+++ b/src/AggregateFunctions/AggregateFunctionIntervalLengthSum.h
@@ -26,12 +26,11 @@ namespace ErrorCodes
     extern const int TOO_LARGE_ARRAY_SIZE;
 }
 
-/**
- * Calculate total length of intervals without intersections. Each interval is the pair of numbers [begin, end];
- * Return Int64 for integral types (UInt/Int*, Date/DateTime) and return Float64 for Float*.
- *
- * Implementation simply stores intervals sorted by beginning and sums lengths at final.
- */
+/** Calculate total length of intervals without intersections. Each interval is the pair of numbers [begin, end];
+  * Returns UInt64 for integral types (UInt/Int*, Date/DateTime) and returns Float64 for Float*.
+  *
+  * Implementation simply stores intervals sorted by beginning and sums lengths at final.
+  */
 template <typename T>
 struct AggregateFunctionIntervalLengthSumData
 {
@@ -46,10 +45,14 @@ struct AggregateFunctionIntervalLengthSumData
 
     void add(T begin, T end)
     {
+        /// Reversed intervals are counted by absolute value of their length.
+        if (unlikely(end < begin))
+            std::swap(begin, end);
+        else if (unlikely(begin == end))
+            return;
+
         if (sorted && !segments.empty())
-        {
             sorted = segments.back().first <= begin;
-        }
         segments.emplace_back(begin, end);
     }
 
@@ -147,7 +150,7 @@ private:
 
         for (size_t i = 1, sz = data.segments.size(); i < sz; ++i)
         {
-            /// Check if current interval intersect with next one then add length, otherwise advance interval end
+            /// Check if current interval intersects with next one then add length, otherwise advance interval end.
             if (cur_segment.second < data.segments[i].first)
             {
                 if constexpr (std::is_floating_point_v<TResult>)
@@ -184,7 +187,7 @@ public:
     {
         if constexpr (std::is_floating_point_v<T>)
             return std::make_shared<DataTypeFloat64>();
-        return std::make_shared<DataTypeInt64>();
+        return std::make_shared<DataTypeUInt64>();
     }
 
     bool allocatesMemoryInArena() const override { return false; }
@@ -225,7 +228,7 @@ public:
         if constexpr (std::is_floating_point_v<T>)
             assert_cast<ColumnFloat64 &>(to).getData().push_back(getIntervalLengthSum<Float64>(this->data(place)));
         else
-            assert_cast<ColumnInt64 &>(to).getData().push_back(getIntervalLengthSum<Int64>(this->data(place)));
+            assert_cast<ColumnUInt64 &>(to).getData().push_back(getIntervalLengthSum<UInt64>(this->data(place)));
     }
 };