Function plan_set_operation 

pub fn plan_set_operation(
    op: &SetOperator,
    left: &SetExpr,
    right: &SetExpr,
    quantifier: &SetQuantifier,
    catalog: &dyn SqlCatalog,
    functions: &FunctionRegistry,
) -> Result<SqlPlan>

Plan a UNION / UNION ALL / INTERSECT / EXCEPT operation.