def set_cursor_tuple_fraction(frac = 1.0)
      @cursor_tuple_fraction ||= @options.fetch(:fraction, 1.0)
      return @cursor_tuple_fraction if frac == @cursor_tuple_fraction
      @cursor_tuple_fraction = frac
      @result = @connection.execute("set cursor_tuple_fraction to  #{frac}")
      frac
    end
Private: Sets the PostgreSQL cursor_tuple_fraction value = 1.0 to assume all rows will be fetched
This is a value between 0.1 and 1.0 (PostgreSQL defaults to 0.1, this library defaults to 1.0)
used to determine the expected fraction (percent) of result rows returned the the caller.
This value determines the access path by the query planner.