In a plant information management system (PIMS, 1), data received from a process device (PCC, 10) is cached by a data logger (2) in blocks. A database controller (4) receives, for each data event, a time string, a value string, and a data quality indicator. The quality indicator is stored by exception only (if "bad"). MSBs of the time string are encoded in a hierarchical database structure of item (process device)--year--day levels. LSBs of the time string, the value string, and quality exceptions are stored as event objects in linked list arrays referenced to a day array object. There is thus no need for an index and the structure provides hierarchical compression.