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, 2 insertions, 1 deletions

diff --git a/sql/field.cc b/sql/field.cc
index e6e4195ba1e..1a0e93c82ac 100644
--- a/sql/field.cc
+++ b/sql/field.cc
@@ -1304,7 +1304,8 @@ Field::Field(char *ptr_arg,uint32 length_arg,uchar *null_ptr_arg,
    field_name(field_name_arg),
    query_id(0), key_start(0), part_of_key(0), part_of_sortkey(0),
    unireg_check(unireg_check_arg),
-   field_length(length_arg), null_bit(null_bit_arg) 
+   field_length(length_arg), null_bit(null_bit_arg), 
+   is_created_from_null_item(FALSE)
 {
   flags=null_ptr ? 0: NOT_NULL_FLAG;
   comment.str= (char*) "";