Explain EXPLAIN QUERY TREE (children 2)
 Set
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 2)
    ExpressionList (children 1)
     Literal Tuple_(UInt64_1, UInt64_2) (alias t)
    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 UInt64_2