Bug#32848: Data type conversion bug in union subselects in MySQL 5.0.38

There were two problems when inferring the correct field types resulting from
UNION queries.
- If the type is NULL for all corresponding fields in the UNION, the resulting 
  type would be NULL, while the type is BINARY(0) if there is just a single 
  SELECT NULL.
- If one SELECT in the UNION uses a subselect, a temporary table is created
  to represent the subselect, and the result type defaults to a STRING type,
  hiding the fact that the type was unknown(just a NULL value).
Fixed by remembering whenever a field was created from a NULL value and pass
type NULL to the type coercion if that is the case, and creating a string field
as result of UNION only if the type would otherwise be NULL.

1 files changed, 4 insertions, 0 deletions

diff --git a/sql/item.cc b/sql/item.cc
index 3177c0fb1e8..5df0f441619 100644
--- a/sql/item.cc
+++ b/sql/item.cc
@@ -6569,6 +6569,8 @@ enum_field_types Item_type_holder::get_real_type(Item *item)
     */
     Field *field= ((Item_field *) item)->field;
     enum_field_types type= field->real_type();
+    if (field->is_created_from_null_item)
+      return MYSQL_TYPE_NULL;
     /* work around about varchar type field detection */
     if (type == MYSQL_TYPE_STRING && field->type() == MYSQL_TYPE_VAR_STRING)
       return MYSQL_TYPE_VAR_STRING;
@@ -6820,6 +6822,8 @@ Field *Item_type_holder::make_field_by_type(TABLE *table)
                          Field::NONE, name,
                          table, get_set_pack_length(enum_set_typelib->count),
                          enum_set_typelib, collation.collation);
+  case MYSQL_TYPE_NULL:
+    return make_string_field(table);
   default:
     break;
   }