Function exact_function 

pub fn exact_function(
    catalog: &dyn RoutineNameCatalog,
    identities: &[RoutineCatalogIdentity],
    name: &str,
    argument_types: &[i64],
    display_types: &[String],
) -> Result<usize, SQLError>

LookupFuncName matches declared input OIDs exactly, ignores procedures, and does not expand defaults or variadics.