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SQLite doesn't have a separate server process; it does all of the work for queries in your process. So it's intrinsically CPU-heavy, and it needs threads to avoid blocking the event loop.

One way to look at is that with a client-server database and an async client library, you have a thread pool in the database server process to do the heavy lifting, and async clients talk to it via TCP. With SQLite, you have that "server" thread pool in the same process instead, and async "clients" talk to it via in-process communication.

Cause the sqlite-lib that python ships isn't async, and sqlite itself usually doesn't give an async API.

Python's asyncio is single threaded. If you didn't send them into a different thread, the entire event loop would block, and it would degenerate to a fully synchronous single threaded program with additional overhead.