command deterministic subqueries without from clauses should be merged if possible

Defect Tracker #24752: command deterministic subqueries without from clauses should be merged if possible:

i.e. select e1, a from pm1.g1, (select 1 a) x <=> select e1, 1 from pm1.g1
i.e. select e1, (select 1 a) from pm1.g1 <=> select e1, 1 from pm1.g1