[all0c]

(I'm from the FDB team and work on the Record Layer.) As ryanworl's excellent answer suggests, the FoundationDB key value does not support secondary indexing on its own. It is strictly an ordered store mapping byte-array keys to byte-array values.

Secondary indexing is a core feature of the Record Layer, though! It includes a variety of secondary index types. The simplest are implemented using essentially the same strategy as ryanworl outlines (with more details on how that index works available in the key-value store documentation: https://apple.github.io/foundationdb/simple-indexes.html). And index updates are all entirely transactional (i.e., as the index update happens in the same transaction as record insertion, they are always consistent and up-to-date). However, all of that happens behind the scenes. The API presented to the user only asks for what record to save (update or insert), and then the Record Layer updates the appropriate indexes using a user-provided schema. Importantly, the Record Layer also supports handling the various stages of index maintenance (e.g., deleting an index's data after removing it from the schema or filling in data from existing records after an index is added). More can be found within the Record Layer overview: https://foundationdb.github.io/fdb-record-layer/Overview.htm...