About data retrieval

Through the Data Retrieval subsystem, AVEVA Historian receives SQL queries from clients, locates the requested data, performs necessary processing, and then returns the results.

For configuration and event data, retrieval is made possible by normal SQL queries, because these types of data are stored in SQL Server database tables. Historical data, however, must be retrieved from history blocks and then sent to clients as if it is stored in SQL Server tables.

To accomplish retrieval from both data repositories, the Data Retrieval subsystem includes:

• An implementation of a SQL Server data provider.
  This determines whether the requested data is saved in SQL Server tables or in history blocks.

• Retrieval subsystem.
  This subsystem is responsible for extracting the requested data from the history blocks and presenting to the AVEVA Historian OLE DB provider as "virtual" history tables.

• A set of SQL Server extensions.
  These are implemented as columns in the history tables. You can use these extensions to specify the nature of the rowset that is returned, such as the number of rows returned, the resolution of the data, or the retrieval mode.

For more information on data storage, see Manage data storage.