Hmm, perhaps I don't fully understand what you are trying to do.
You want to do UNION with 5 tables and then use the union as a subquery. But could you please give me a more concrete example, how the subquery should look like?
The example, which I sent you, is also union with 5 tables used as subquery and I was able to create this with query builder. But maybe this is not exactly what you want. The only problem, which I see in my example, is that the unions are in the nested brackets (always only two tables). But I don't see this as a big problem. Am I wrong?
As I said I don't know, what might be causing the freezing, but I had a theory, that this might be caused by disabled cache.
Libor