Thanks! Yup, it's not always practical to make a huge number of queries when you expect many of them to come back empty. Instead, we first perform private lookups against a Bloom filter, to find out which keys actually hold data (e.g. messages). Then, we privately retrieve only the useful keys.

The Bloom filter is also served over Blyss, so the server still learns nothing about which keys you're interested in. We implemented this system for our private password checker, which tests passwords against almost a billion breached credentials: https://playground.blyss.dev/passwords