Fixed bug #29251.

Sometimes special 0 ENUM values was ALTERed to normal
empty string ENUM values.

Special 0 ENUM value has the same string representation
as normal ENUM value defined as '' (empty string).
The do_field_string function was used to convert
ENUM data at an ALTER TABLE request, but this
function doesn't care about numerical "indices" of
ENUM values, i.e. do_field_string doesn't distinguish
a special 0 value from an empty string value.

A new copy function called do_field_enum has been added to
copy special 0 ENUM values without conversion to an empty
string.


sql/field_conv.cc:
  Fixed bug #29251.
  The Copy_field::get_copy_func method has been modified to
  return a pointer to the do_field_enum function if a conversion
  between two columns of incompatible enum types is required.
  The do_field_enum function has been added for the correct
  conversion of special 0 enum values.
mysql-test/t/type_enum.test:
  Updated test case for bug #29251.
mysql-test/r/type_enum.result:
  Updated test case for bug #29251.

1 files changed, 16 insertions, 1 deletions

diff --git a/sql/field_conv.cc b/sql/field_conv.cc
index 59b550572c3..4a7935a4be7 100644
--- a/sql/field_conv.cc
+++ b/sql/field_conv.cc
@@ -311,6 +311,15 @@ static void do_field_string(Copy_field *copy)
 }
 
 
+static void do_field_enum(Copy_field *copy)
+{
+  if (copy->from_field->val_int() == 0)
+    ((Field_enum *) copy->to_field)->store_type((ulonglong) 0);
+  else
+    do_field_string(copy);
+}
+
+
 static void do_field_int(Copy_field *copy)
 {
   longlong value=copy->from_field->val_int();
@@ -538,7 +547,13 @@ void (*Copy_field::get_copy_func(Field *to,Field *from))(Copy_field*)
 	  to->real_type() == FIELD_TYPE_SET)
       {
 	if (!to->eq_def(from))
-	  return do_field_string;
+        {
+          if (from->real_type() == MYSQL_TYPE_ENUM &&
+              to->real_type() == MYSQL_TYPE_ENUM)
+            return do_field_enum;
+          else
+            return do_field_string;
+        }
       }
       else if (to->charset() != from->charset())
 	return do_field_string;