In order to ask this question it's important to understand the lifecycle of the data in question. If it is constantly being updated and requires "liveness" (updates are reflected in queries immediately), the simple answer is: yes, you need a database.

But if you have data that is static or effectively static (data that is updated occasionally or batched), then serving via custom file handling can have its place.

If the records are fixed width and sorted on the key value, then it becomes trivial to do a binary search on the mmapped file. It's about as lightweight as could be asked for.