Like BerkeleyDB, and unlike most other key/value stores around today, #LMDB is more than just a simple K/V store. Its support for multiple tables, and txns spanning multiple tables, lets you implement other features like 2ndary indexing, metadata storage, etc., that ordinary K/V stores just can't do. Here, #HarperDB describes how they designed their data model on top of LMDB
https://docs.harperdb.io/docs/technical-details/reference/storage-algorithm