[mattashii]

> Sure it sounds interesting to try a branch of pg with, for example, just the sessions being multithreaded - but then how DOES one forcibly stomp on some session that has grabbed some critical lock without crashing other users' sessions?

Presumably as one does now, through pg_terminate_backend()/pg_cancel_backend().

> Killing off a session's thread inside of a MT'ed session handler without putting any other threads at risk would be the first problem (and an admin is likely to use "ps -Lef" to find the thread ID and then "kill")

ps+kill already puts all of Postgres' processes at risk in Postgres' MP system, because processes that unexpectedly exit may have corrupted shared state, so in those situations PG restarts. MT would not significantly change that.

> Going too crazy with threads can also cause performance issues, since there is overhead - just less than for processes - around thread creation/switching/etc, and is why thread pools are common.

(emphasis mine)

Considering that PostgreSQL currently is a multi-process architecture, surely replacing the Process primitive with the Thread primitive will reduce the overhead of connection backends, all else being equal.