Explain EXPLAIN SYNTAX (children 1)
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 3)
    ExpressionList (children 2)
     Function plus (children 1)
      ExpressionList (children 2)
       Literal UInt64_1
       Function greatest (children 1)
        ExpressionList (children 2)
         Identifier x1
         Literal UInt64_1
     Identifier x2
    TablesInSelectQuery (children 1)
     TablesInSelectQueryElement (children 1)
      TableExpression (children 1)
       TableIdentifier test
    ExpressionList (children 2)
     Literal UInt64_1
     Literal UInt64_2