Delta retrieval

Delta retrieval, or retrieval based on exception, is the retrieval of only the changed values for a tag(s) for the given time interval. That is, duplicate values are not returned. It works with all types of tags.

Delta retrieval always produces a rowset comprised of only rows that are actually stored on the historian; that is, a delta query returns all of the physical rows in history for the specified tags, over the specified period, minus any duplicate values. If there is no actual data point at the start time, the last data point before the start time is returned.

Delta retrieval is the default mode for discrete and string tables and from the History table.