Thanks, it was $sth->{"mysql_use_result"} = 1; that I needed!

The right indexes are all in place (and EXPLAIN SELECT agrees), there are just a huge number of rows in the result set.

Now here's the other weird thing, which doesn't matter for my real project but is still unusual. When I make this change, the test program I posted above works better, but now it hangs for a long time during the finish. That surprises me; I would expect it to cancel the query and return immediately. Anybody know what's going on?