Notes and Restrictions

The following points should be regarded when using RDB compression:

  • The configuration of RDB compressions is only possible with a local Oracle client and queryRDBdirect = 1 must be set in the config file. This restriction does not apply for querying compressed values.

  • In order to use the RDB compression, the not compressed WinCC OA values must be archived.

  • The RDB compression can only be used for data point elements of the type uint, int or float.

  • In WinCC OA the compressed RDB values cannot be displayed "online", i.e. a permanent refresh of the values in the trend and the event screen does not take place. To see the current compressed data, the trend / event screen must be closed and reopened. Historical queries are supported.

  • An access to the compressed information is possible via the trend and the functions dpGetPeriod(), dpGetAsynch() and dpQuery() (with TIMERANGE) with the _offline.._compressions attribute.

  • In one SQL statement only one "_value" config is allowed. That means that when querying compressed values using dpQuery() only one "_value" config can be queried in the same function.

  • Value changes, which are received after a calculation time (original and correction values) are not taken into account excepted a delay was defined for the interval.

  • Only compressed original values are stored - no online, offline values or status bits.

  • Interim values of compressions are not calculated.

  • In the standard WinCC OA ASCII import and export the configured compression intervals, steps and calculations are not taken into account. For this purpose the panel provided by the RDB compression has to be used - see ASCII Import and Export of a RDB Compression Configuration.

  • If a base compression is inactivated, the subordinated compressions will not be deactivated. That means that the subordinated compression is still calculated.

  • It is to consider that, when saving values out of the defined value range (e.g. NaN, +/- infinite,...) a distortion of the RDB compressed values (AVG, MIN, MAX,...) will occur, due to the compressed values that will also be out of the defined range. To prevent this distortion, it is recommended to define a WinCC OA value range and to handle values out of the defined range with a default value.