Fabio

>I think that you have not understood where the problem is.

I understood exactly what you are calling a bug.

>Are you happy that these two queries return a final cursor with different column name?

As I said before I wouldn't use inconsistent column names in my query and complain when the SQL query engine decides to use one or the other depending on how it accomplishes the task.

>

>* this return X
>SELECT F1 X FROM T1;
>UNION ALL;
>SELECT F2 Y FROM T2
>
>* this return Y
>SELECT F1 X FROM T1;
>UNION ;
>SELECT F2 Y FROM T2
>

I'd write the queries like this and be happy all day long that I got consistent results:

* this returns X
SELECT F1 X FROM T1;
UNION ALL;
SELECT F2 FROM T2

* this returns X
SELECT F1 X FROM T1;
UNION ;
SELECT F2 FROM T2

* this returns X
SELECT F1 X FROM T1;
UNION ALL;
SELECT F2 X FROM T2

* this returns X
SELECT F1 X FROM T1;
UNION ;
SELECT F2 X FROM T2

It just doesn't make much sense that you'd give alternate aliases (X in one statement and Y in another) to the columns between the select statements. For non-trivial queries it would be much harder for another developer following behind you to understand/modify/debug/ the SQL code.