Gregory Stark <[EMAIL PROTECTED]> writes: > "Manoel Henrique" <[EMAIL PROTECTED]> writes: >> Yes, I'm relying on the assumption that backwards scan has the same cost as >> forward scan, why shouldn't it?
> Because hard drives only spin one direction Good joke, but to be serious: we expect that forward scans will result in the kernel doing read-ahead, which will allow overlapping of CPU work to process one page with the I/O to bring in the next page. A backwards scan will get no such overlapping and thus be up to 2X slower, unless the kernel is smart enough to do read-ahead for descending-order read requests. Which seems not too probable. A fairly typical kernel behavior is that read-ahead is triggered by successive read() requests without any intervening seek(), and this is impossible for a backward scan. (Yes, we do optimize out the seek calls in a forward scan. IIRC it's done in fd.c.) regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers