a) if you really need to run hundreds or thousands queries per second against external
database i'd say you have a problems with design\architecture, sorry

That's exactly the sort of opinion based on real experience of trying it that I was after, thanks.

b) you still don't read docs carefully: just run any query against external db using COMMON
transaction and all your subsequent external queries against same external db will use the
same external connection (for both COMMON and AUTONOMOUS tx). Until local transaction
alive, of course.

Ah! - you're saying that a workaround to AUTONOMOUS closing the connection is to perform a do-nothing COMMON statement first, and then do the AUTONOMOUS ones. Yes, one can deduce that from the documentation, but it hardly makes it clear that this is a deliberate feature! - and I must say it does rather look slightly more like an accident of implementation than a deliberate feature to me.

-- 
Tim Ward