SelectWithUnionQuery (children 1)
 ExpressionList (children 1)
  SelectQuery (children 3)
   ExpressionList (children 2)
    Function toLowCardinality (children 1)
     ExpressionList (children 1)
      Function materialize (children 1)
       ExpressionList (children 1)
        Literal \'a\' (alias key)
    Literal \'b\' (alias value)
   ExpressionList (children 1)
    Identifier key
   Set
