If have a query of the form
SELECT a FROM (
SELECT a FROM (
SELECT a FROM ds.t1
) u
) t GROUP BY a

Teiid correctly optimizes the inner SELECT to SELECT 1 FROM ds.t1. However, if the inner subselect is a union:
SELECT a FROM (
SELECT a FROM (
SELECT a FROM ds.t1 UNION
SELECT a FROM ds.t2
) u
) t GROUP BY a

Teiid does not optimize to
SELECT 1 FROM ds.t1 UNION
SELECT 1 FROM ds.t2
as I would expect.

See attached plan