... "what's no longer needed" is, the police will strenuously insist, an empty set. You never know when some tidbit of information from 1993 is suddenly going to turn out to be the key to a current investigation.
Personally, I think our only plausible salvation from police abuse of this system is police bureaucracy, which I would model as follows:
- Every time an officer runs a query on a database, force them to complete a 'reason' field referencing a case they're working on.
- Insist that results should be recorded only on dedicated police equipment (if that means buying everyone a brand-new top-of-the-line tablet every year, then fine, that's a small price to pay). Copying the results to any other media, including paper napkins or Post-it notes, should be a serious disciplinary offence.
- Program the devices to automatically delete any database query results after, let's say, 24 hours, unless the officer flags them as "interesting".
- Periodically, conduct an audit where you randomly review a handful of cases on the officer's tablet, and require them to justify the results they've flagged as "interesting".