diff --git a/dbms/src/DataStreams/FillingBlockInputStream.cpp b/dbms/src/DataStreams/FillingBlockInputStream.cpp
index aefcb4bf982..6c0ee82c690 100644
--- a/dbms/src/DataStreams/FillingBlockInputStream.cpp
+++ b/dbms/src/DataStreams/FillingBlockInputStream.cpp
@@ -227,6 +227,7 @@ Block FillingBlockInputStream::readImpl()
             if (!filling_row.getFillDescription(i).fill_from.isNull() &&
                 less(filling_row.getFillDescription(i).fill_from, (*old_fill_columns[i])[0], filling_row.getDirection(i)))
             {
+                /// Insert filling row, if it's less than first row in block, because of set 'fill_from' value.
                 filling_row.initFromDefaults(i);
                 insertFromFillingRow(res_fill_columns, res_other_columns, filling_row);
                 break;
diff --git a/dbms/src/DataStreams/FillingBlockInputStream.h b/dbms/src/DataStreams/FillingBlockInputStream.h
index 5097fe5c6b8..b7079fba2cc 100644
--- a/dbms/src/DataStreams/FillingBlockInputStream.h
+++ b/dbms/src/DataStreams/FillingBlockInputStream.h
@@ -10,7 +10,7 @@ class FillingRow
 public:
     FillingRow(const SortDescription & sort_description);
 
-    /// Generates next row according to fill from, to and step values.
+    /// Generates next row according to fill 'from', 'to' and 'step' values.
     /// Returns true, if generated row less than to_row in terms of sorting order.
     bool next(const FillingRow & to_row);