Anyone interested this topic would also be interested in the discussion at https://news.ycombinator.com/item?id=32285435.
mvsqlite might be able to handle cross-database transactions in the storage layer(=mvsqlite's mvstore backed by FoundationDB's fully serializable transactions) rather than sqlite's native one so I think there should be no theoretical limit due to SQLITE_MAX_ATTACHED.
Update:
But yeah, mvsqlite as of today seems to be tied to one sqlite per mvstore so it's attaching databases to the sqlite instance is the only way to deal with multi databases. So, it will naturally be affected by SQLITE_MAX_ATTACHED.
Perhaps a potential big idea would be to have a sqlite instance per database and a "proxy" layer above sqlite instances to (1)obtain/set a global transaction ID per multi-db transaction and (2)redirect queries to each involved sqlite databse, while the serializability is guaranteed in the FoundationDB.
Update:
But yeah, mvsqlite as of today seems to be tied to one sqlite per mvstore so it's attaching databases to the sqlite instance is the only way to deal with multi databases. So, it will naturally be affected by SQLITE_MAX_ATTACHED.
Perhaps a potential big idea would be to have a sqlite instance per database and a "proxy" layer above sqlite instances to (1)obtain/set a global transaction ID per multi-db transaction and (2)redirect queries to each involved sqlite databse, while the serializability is guaranteed in the FoundationDB.