According to https://www.postgresql.org/docs/9.1/static/protocol-flow.html

  The Describe message (statement variant) specifies the name of an
  existing prepared statement (or an empty string for the unnamed
  prepared statement). The response is a ParameterDescription message
  describing the parameters needed by the statement, followed by a
  RowDescription message describing the rows that will be returned when
  the statement is eventually executed (or a NoData message if the
  statement will not return rows)