>> I'm wondering why --- doesn't seem like it should take 6400msec to fetch
>> 646 rows, unless perhaps the data is just horribly misordered relative
>> to the index. Which may in fact be the case ...
Hmm, actually I still don't understand why it takes 6400 ms to fetch the
rows. As far as I can see the index used is "covering" so that real row
lookups shouldn't be necessary. Also, only the the random_numbers
induces by questions with status = 1 should be considered - and this
part is a relatively small subset.
In general, I don't understand why the query is so I/O dependant as it
apparently is.