When executing queries like:
"SELECT COALESCE(name, 'unknown') as name_out FROM PCOLLECTION"
(input 'name' is nullable)
There is a need to know whether the result is nullable or not (in this case - not). During validation stage "COALESCE" is being transformed (via SqlValidatorImpl.performUnconditionalRewrites) into a "CASE" statement, which currently does not determine nullability of a result.