Use case

Allow the ugliness of
SELECT NOW, CURRENT_TIMESTAMP
to work as well as a normal, beautiful query:
SELECT now()

Describe the solution you'd like

Function factory should support obtaining a property "does the function allow omitting the arguments list".
If an identifier is not resolved in a query, the function factory will be queried for the existing function, and we'll check that property. We will provide a good exception message if the function with the specified name exists but does not allow omitting the parenthesis.