MDEV-19283 Move the code from Field_str::is_equal() to Field_string::is_equal()

sql/field.cc

@@ -2436,6 +2436,15 @@ void Field_null::sql_type(String &res) const
 }
 
 
+uint Field_null::is_equal(Create_field *new_field)
+{
+  DBUG_ASSERT(!compression_method());
+  return new_field->type_handler() == type_handler() &&
+         new_field->charset == field_charset &&
+         new_field->length == max_display_length();
+}
+
+
 /****************************************************************************
   Field_row, e.g. for ROW-type SP variables
 ****************************************************************************/
@@ -7069,14 +7078,10 @@ int Field_str::store(double nr)
   return store(buff, (uint)length, &my_charset_numeric);
 }
 
-uint Field::is_equal(Create_field *new_field)
-{
-  return new_field->type_handler() == type_handler();
-}
-
 
-uint Field_str::is_equal(Create_field *new_field)
+uint Field_string::is_equal(Create_field *new_field)
 {
+  DBUG_ASSERT(!compression_method());
   if (new_field->type_handler() != type_handler())
     return IS_EQUAL_NO;
   if (new_field->length < max_display_length())

sql/field.h

@@ -1504,7 +1504,7 @@ class Field: public Value_source
     of a table is compatible with the old definition so that it can
     determine if data needs to be copied over (table data change).
   */
-  virtual uint is_equal(Create_field *new_field);
+  virtual uint is_equal(Create_field *new_field)= 0;
   /* convert decimal to longlong with overflow check */
   longlong convert_decimal2longlong(const my_decimal *val, bool unsigned_flag,
                                     int *err);
@@ -1868,7 +1868,6 @@ class Field_str :public Field {
   my_decimal *val_decimal(my_decimal *);
   bool val_bool() { return val_real() != 0e0; }
   virtual bool str_needs_quotes() { return TRUE; }
-  uint is_equal(Create_field *new_field);
   bool eq_cmp_as_binary() { return MY_TEST(flags & BINARY_FLAG); }
   virtual uint length_size() { return 0; }
   double pos_in_interval(Field *min, Field *max)
@@ -2640,6 +2639,7 @@ class Field_null :public Field_str {
   my_decimal *val_decimal(my_decimal *) { return 0; }
   String *val_str(String *value,String *value2)
   { value2->length(0); return value2;}
+  uint is_equal(Create_field *new_field);
   int cmp(const uchar *a, const uchar *b) { return 0;}
   void sort_string(uchar *buff, uint length)  {}
   uint32 pack_length() const { return 0; }
@@ -3563,6 +3563,7 @@ class Field_string :public Field_longstr {
   int cmp(const uchar *,const uchar *);
   void sort_string(uchar *buff,uint length);
   void sql_type(String &str) const;
+  uint is_equal(Create_field *new_field);
   virtual uchar *pack(uchar *to, const uchar *from,
                       uint max_length);
   virtual const uchar *unpack(uchar* to, const uchar *from,