You're right, and I was fuzzy about what I meant. I didn't mean (although wasn't clear) that QUERY would have to return the whole result set over time - clearly

You're right, and I was fuzzy about what I meant. I didn't mean (although wasn't clear) that QUERY would have to return the whole result set over time - clearly that's beyond the scope of HTTP's definition of idempotency. However, because of the existence of isolation and locking concerns in databases, even fairly simple queries are not idempotent. RFC2616 goes to some effort to (fuzzily, unfortunately) talk about sequences of operations, which would be useful here.