In a database, you don't really delete a record, you simply mark it deleted. Depending on the system you are using, that disk space might then be re-used for a new record, or it might sit, wasting space. In that sense, you might need to re-pack a database on occassion.

Convenional defragmentation, on the other hand, is not only a filesystem concern, but a Microsoft concern. It does not apply to Unix file systems. That's why you have to de-frag MS file systems, but there's no such utility for Unix.