Retrieving Data

To retrieve data from the historian server, it is necessary to have a physical connection to it. You can ensure that by checking the ConnectedToServer property of the HistorianConnectionStatus instance.

The Historian SDK allows for data retrieval in the same way as using Transact-SQL from the History, AnalogSummary, and StateSummary tables of the Runtime database. In fact, the managed HCAL retrieval mechanism bypasses the AVEVA Historian OLE DB provider level and goes directly to the Historian Retrieval service. That is why using the Historian SDK for retrieval is expected to be faster and more efficient than using the Microsoft ADO interface for retrieving the same data from the Microsoft SQL Server.

You may still consider using Microsoft ADO for retrieval if you need richer functionality related to filtering and joining data provided by the Microsoft SQL Server infrastructure.

If you compare retrieval results with high decimal point resolutions produced by the Historian SDK and Microsoft SQL Server, you may notice some slight differences, because while the Historian Managed HCAL infrastructure retrieves data values directly from the Historian Retrieval Service, the OLE DB provider layer may perform additional value conversions along the way.