a database management system includes a first data storage structure for storing timestamped counter data entries for each of devices in an unordered manner and such that each counter data entry is assigned a uniquely identifiable storage address ; a second data storage structure for storing and mapping between : address data identifying the storage addresses of counter data entries , excluding those which are null or empty , timestamps corresponding to the counter data entries , and device identifiers identifying the devices to which the counter data entries correspond ; and data access means for , in response to receiving a request for data in respect of a specified time period and at least one of the devices , using the second data storage structure to identify , for the or each specified device , the storage addresses of the temporally first and last counter data entries falling within the specified time period and not null or empty .