Explain EXPLAIN QUERY TREE (children 2)
 Set
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 2)
    ExpressionList (children 1)
     Function CAST (alias t) (children 1)
      ExpressionList (children 2)
       Literal \'(1, 2)\'
       Literal \'Tuple(a UInt32, b UInt32)\'
    ExpressionList (children 2)
     Function tupleElement (children 1)
      ExpressionList (children 2)
       Identifier t
       Literal UInt64_1
     Function tupleElement (children 1)
      ExpressionList (children 2)
       Identifier t
       Literal \'b\'