This page is about data source / data storage backends. (Do not use the abbreviation "DS" for this.)
The missing feature is the ability to find the previous record (in sorted order) from a starting position in the database that does not exist. i.e., if there's a record g, but no h, and you ask for first record less than h, that doesn't work. (However, if h does exist, then you can go backwards from there to g.)
Apparently BDB has that implemented in java, but it doesn't exist in the C++ api.
TODO: what is the name of the function call in Java?
TODO: is there an upstream ticket to get this in the C++ API?