> If I stop the Firebird Engine before the crash, I get the following
> dataset result:
>
> COLUMN0, COLUMN1, SUM
> 0, <null>, <null>
> 0, <null>, <null>
> 0, <null>, <null>
> ....
>
>
> Hundreds of similar rows.
>
> Is it a bug of Firebird?
>
> Thank you

No it is a bug in your logic that assumes that NULL = NULL

NULL does not equal NULL
Therefore, every (0, NULL) combination would be considered unique.

In fact I am not sure why

select NULL, sum(*)
from customers
group by 1

would work (although it does, logically it shouldn't). Perhaps that
is a bug.

Read this NULL guide for more info.
http://www.ibphoenix.com/main.nfs?a=ibphoenix&page=ibp_firebird_null

By the way it is possible to use first 1 syntax to stop it going to
infinity, but still I would be looking for some other way of the
wizard handling this.

Hope that helps a little.
Adam