Explain EXPLAIN QUERY TREE (children 2)
 Set
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 2)
    ExpressionList (children 3)
     Identifier id
     Function isNull (children 1)
      ExpressionList (children 1)
       Identifier left.n
     Function isNull (children 1)
      ExpressionList (children 1)
       Identifier right.n
    TablesInSelectQuery (children 2)
     TablesInSelectQueryElement (children 1)
      TableExpression (children 1)
       TableIdentifier t_func_to_subcolumns (alias left)
     TablesInSelectQueryElement (children 2)
      TableExpression (children 1)
       Subquery (alias right) (children 1)
        SelectWithUnionQuery (children 1)
         ExpressionList (children 2)
          SelectQuery (children 1)
           ExpressionList (children 2)
            Literal UInt64_1 (alias id)
            Literal \'qqq\' (alias n)
          SelectQuery (children 1)
           ExpressionList (children 2)
            Literal UInt64_3 (alias id)
            Literal \'www\'
      TableJoin (children 1)
       ExpressionList (children 1)
        Identifier id
