Not found, every time. The database still did the work.
Real customers wait behind the pointless lookups.
It says definitely not, or maybe.
It never wrongly says no to something that is there.
It is tiny, because it stores bits, not the items.
Reach for it when
When most lookups find nothing, like bots guessing ids, and each database check is slow. Not when most lookups succeed.
The catch
It sometimes says maybe for ids that do not exist, wasting a lookup, and plain ones cannot delete items.
Say it in an interview“I would put a Bloom filter in front of the database: a no is certain, a maybe still gets checked, so false positives only cost one extra lookup.”