> 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.