Added a function comment to Field_varstring::mark_unused_memory_as_defined()

sql/field.cc

@@ -7867,6 +7867,19 @@ bool Field_varstring::send(Protocol *protocol)
 
 
 #ifdef HAVE_MEM_CHECK
+
+/*
+  Mark the unused part of the varstring as defined.
+
+  This function is only used be Unique when calculating statistics.
+
+  The marking is needed as we write the whole tree to disk in case of
+  overflows.  For using or comparing values the undefined value part
+  is never used. We could also use bzero() here, but it would be
+  slower in production environments.
+  This function is tested by main.stat_tables-enospc
+*/
+
 void Field_varstring::mark_unused_memory_as_defined()
 {
   uint used_length __attribute__((unused)) = get_length();