diff --git a/src/Functions/in.cpp b/src/Functions/in.cpp
index 70c669fea28..3448e09b990 100644
--- a/src/Functions/in.cpp
+++ b/src/Functions/in.cpp
@@ -130,6 +130,8 @@ public:
         else
             columns_of_key_columns.insert(left_arg);
 
+        /// Replace single LowCardinality column to it's dictionary if possible.
+        /// It is good enough optimization, but it does not use LowCardinality cache.
         ColumnPtr lc_indexes = nullptr;
         if (columns_of_key_columns.columns() == 1)
         {