> would happen the same in a single-threaded event loop

Well...if you implemented a relational DBMS server without using threads. To my knowledge, no such DBMS exists, so the distinction seems rather academic.

> Btw if you really want consistent multi reads, some DBMSes support setting a read timestamp, but the common ones don't.

Could you elaborate? I can't say I heard of that mechanism. Perhaps you are referring to something like Oracle flashback queries or SQL Server temporal tables?

Normally, I'd use MVCC-based "snapshot" transaction isolation for consistency between multiple queries, though they would need to be executed serially.

I was talking about the client side here, which is maybe a web backend. If it's using threads, at least the connection pool will be thread-safe. If it's event loop, N/A.

If you want to look at the DBMS itself, well typically there's a separate process per connection, but say it uses threading instead... It'd be thread-safe too. You aren't hitting UB by doing concurrent xacts.

Snapshot xact is what I was thinking about. Not sure about Oracle, but in Spanner they can be parallel.