BloomFilters make for good indices, too. My first encounter with them was from the BigTable paper.
We used it in front of a Radix Tree that at places had too much depth. The filter took something like 2MiB at 7% false-positive rate vs 100+ MiB for the Radix Tree. We later bitmap-compressed the Radix Tree [0] to under 5MiB at the cost of making the lookups very very slow (P99 at 4ms, P50 at 1ms compared to P99 at 400ns before, iirc) but added a TinyLFU-based [1] lean-HAMT [2] cache of 512KiB in the front to speed lookups of popular items and a Robinhood cache [3] of 256KiB to take care of the long-tail KVs.