MDEV-9234 Add Type_handler::union_element_finalize()

sql/sql_type.cc

@@ -7890,6 +7890,19 @@ bool Type_handler_string_result::Item_eq_value(THD *thd,
 }
 
 
+/***************************************************************************/
+
+bool Type_handler_string_result::union_element_finalize(const Item * item) const
+{
+  if (item->collation.derivation == DERIVATION_NONE)
+  {
+    my_error(ER_CANT_AGGREGATE_NCOLLATIONS, MYF(0), "UNION");
+    return true;
+  }
+  return false;
+}
+
+
 /***************************************************************************/
 
 void Type_handler_var_string::

sql/sql_type.h

@@ -3356,6 +3356,14 @@ class Type_handler
   virtual Field *make_conversion_table_field(TABLE *TABLE,
                                              uint metadata,
                                              const Field *target) const= 0;
+  /*
+    Performs the final data type validation for a UNION element,
+    after the regular "aggregation for result" was done.
+  */
+  virtual bool union_element_finalize(const Item * item) const
+  {
+    return false;
+  }
   // Automatic upgrade, e.g. for ALTER TABLE t1 FORCE
   virtual void Column_definition_implicit_upgrade(Column_definition *c) const
   { }
@@ -4580,6 +4588,7 @@ class Type_handler_string_result: public Type_handler
   void sortlength(THD *thd,
                   const Type_std_attributes *item,
                   SORT_FIELD_ATTR *attr) const;
+  bool union_element_finalize(const Item * item) const;
   bool Column_definition_prepare_stage1(THD *thd,
                                         MEM_ROOT *mem_root,
                                         Column_definition *c,

sql/sql_union.cc

@@ -1085,12 +1085,12 @@ bool st_select_lex_unit::prepare(TABLE_LIST *derived_arg,
 
     while ((type= tp++))
     {
-      if (type->cmp_type() == STRING_RESULT &&
-          type->collation.derivation == DERIVATION_NONE)
-      {
-        my_error(ER_CANT_AGGREGATE_NCOLLATIONS, MYF(0), "UNION");
+      /*
+        Test if the aggregated data type is OK for a UNION element.
+        E.g. in case of string data, DERIVATION_NONE is not allowed.
+      */
+      if (type->type_handler()->union_element_finalize(type))
         goto err;
-      }
     }
     
     /*