diff --git a/dbms/src/Common/HashTable/FixedHashTable.h b/dbms/src/Common/HashTable/FixedHashTable.h
index 2deeb40ac91..0ad56f354b9 100644
--- a/dbms/src/Common/HashTable/FixedHashTable.h
+++ b/dbms/src/Common/HashTable/FixedHashTable.h
@@ -263,6 +263,7 @@ public:
 
 
 public:
+    /// The last parameter is unused but exists for compatibility with HashTable interface.
     void ALWAYS_INLINE emplace(Key x, iterator & it, bool & inserted, size_t /* hash */ = 0)
     {
         it = iterator(this, &buf[x]);