Comments regarding zero values in clearable hash table

src/Common/HashTable/ClearableHashSet.h

@@ -10,6 +10,10 @@
   * Instead of this class, you could just use the pair (version, key) in the HashSet as the key
   * but then the table would accumulate all the keys that it ever stored, and it was unreasonably growing.
   * This class goes a step further and considers the keys with the old version empty in the hash table.
+  *
+  * Zero values note:
+  * A cell in ClearableHashSet can store a zero values as normal value
+  * If its version is equal to the version of the set itself, then it's not considered as empty even key's value is zero value of the corresponding type
   */
 
 

src/Common/HashTable/HashTableKeyHolder.h

@@ -88,7 +88,11 @@ inline StringRef & ALWAYS_INLINE keyHolderGetKey(DB::ArenaKeyHolder & holder)
 
 inline void ALWAYS_INLINE keyHolderPersistKey(DB::ArenaKeyHolder & holder)
 {
-    // Hash table shouldn't ask us to persist a zero key
+    // Normally, our hash table shouldn't ask us to persist a zero key,
+    // but it can happened in the case of clearable hash table (ClearableHashSet, for example).
+    // The clearable hash table doesn't use zero storage and
+    // distinguishes empty keys by using cell version, not the value itself.
+    // So, when an empty StringRef is inserted in ClearableHashSet we'll get here key of zero size.
     // assert(holder.key.size > 0);
     holder.key.data = holder.pool.insert(holder.key.data, holder.key.size);
 }