Explain EXPLAIN PIPELINE (children 1)
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 6)
    ExpressionList (children 1)
     Function sum (children 1)
      ExpressionList (children 1)
       Identifier b
    TablesInSelectQuery (children 1)
     TablesInSelectQueryElement (children 1)
      TableExpression (children 1)
       TableIdentifier pr_t
    ExpressionList (children 1)
     Identifier a
    ExpressionList (children 1)
     OrderByElement (children 1)
      Identifier a
    Literal UInt64_500
    Literal UInt64_5
