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When data of the external historical type is requested from the database, the most recently available data over that whole period is returned. If the data for that period was imported piecewise, then the individual pieces will be merged prior to the data being returned. An example is given in Figure 1 where data is imported sequentially. Each data imported/edited is indicated using a different line style. At the request for the complete series (a) the most recent data available over the complete period is merged and returned. The data imported at 12:00 partially overlaps that imported at 10:00. As the 12:00 data is the most recent, it will persist in the complete series. A manual edit may be done (or interpolation) to fill the gap in the data. This will be returned in a subsequent request for the complete series. Although a complete series is returned, the data is stored as it is imported, including a time stamp indicating when the import happened. If at a later stage the data available at directly preceding the manual edit is requested, then the additional data will not be included in the complete series.
Figure 1 Schematic representation of data imported as external historical
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