1) User has created a view MyView containing UNION all of two other views, eg "SELECT * FROM View1 UNION ALL SELECT * FROM View 2".
2) Then a user query containing a searched CASE type expression is submitted against the view, eg "SELECT max(col) FROM MyView group by CASE WHEN StringNum IS NULL THEN 0 ELSE 1 END"
3) With current planner the CASE expression / group by is not pushed down to the individual sources. The enhancement request is to allow the pushdown.