Backmerge of BUG#12997905

sql/field.h

@@ -2128,6 +2128,23 @@ class Copy_field :public Sql_alloc {
   uchar *from_null_ptr,*to_null_ptr;
   my_bool *null_row;
   uint	from_bit,to_bit;
+  /**
+    Number of bytes in the fields pointed to by 'from_ptr' and
+    'to_ptr'. Usually this is the number of bytes that are copied from
+    'from_ptr' to 'to_ptr'.
+
+    For variable-length fields (VARCHAR), the first byte(s) describe
+    the actual length of the text. For VARCHARs with length 
+       < 256 there is 1 length byte 
+       >= 256 there is 2 length bytes
+    Thus, if from_field is VARCHAR(10), from_length (and in most cases
+    to_length) is 11. For VARCHAR(1024), the length is 1026. @see
+    Field_varstring::length_bytes
+
+    Note that for VARCHARs, do_copy() will be do_varstring*() which
+    only copies the length-bytes (1 or 2) + the actual length of the
+    text instead of from/to_length bytes. @see get_copy_func()
+  */
   uint from_length,to_length;
   Field *from_field,*to_field;
   String tmp;					// For items

sql/field_conv.cc

@@ -707,7 +707,7 @@ Copy_field::get_copy_func(Field *to,Field *from)
         if (((Field_varstring*) to)->length_bytes !=
             ((Field_varstring*) from)->length_bytes)
           return do_field_string;
-        if (to_length != from_length)
+        else
           return (((Field_varstring*) to)->length_bytes == 1 ?
                   (from->charset()->mbmaxlen == 1 ? do_varstring1 :
                                                     do_varstring1_mb) :

sql/sql_select.cc

@@ -10647,6 +10647,9 @@ create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields,
   if (open_tmp_table(table))
     goto err;
 
+  // Make empty record so random data is not written to disk
+  empty_record(table);
+
   thd->mem_root= mem_root_save;
 
   DBUG_RETURN(table);