Good to see some more activity in embedded data space. Earlier used BerkleyDB [1], msql [2] (from where MySQL was built), Postgres 4.2 [3] and now settled on SQLite for all our embedded database needs. When need performance at present just use SQLite in memory db.
Will give Sled a try when it reaches 1.0.0 as at present SQLite serves our embedded requirements.
Make sure to check out lmdb if you haven’t. It’s not relational and has an extremely anemic API but it makes a great low-level building block. Sled is similar I’m that regard, very different from the others you have named.
My company uses LMDB extensively. LevelDB is a LSM database. LSM is generally better for write workloads over BTree DBs like LMDB and sled. LMDB also has a single writer restriction.
Will give Sled a try when it reaches 1.0.0 as at present SQLite serves our embedded requirements.
[1] https://launchpad.net/berkeley-db
[2] https://hughes.com.au/products/msql/
[3] https://dsf.berkeley.edu/postgres.html