I'm curious, why wouldn't sqlite cut it? Granted, I'm not a blockchain afficiando, but I have several sqlite DBs that run a couple hundred gigabytes a piece that work quite well.
The difference between SQLite and Postgres would be that more than one client can connect over the network to PG (or most any other sql db out there really), while SQLite is meant to be embedded, which, if you already have LevelDB working (as is the case in Bitcoin Core), wouldn't be all that different.
What does that mean? Like any RDBMS, SQLite is interfaced with in SQL, but can store and process binary data just fine.