On Fri, Aug 8, 2014 at 5:54 AM, Robert Haas <robertmh...@gmail.com> wrote: > Well, I'm not sure why you're having a hard time imagining it. > Presorted input is a common case in general; that's why we have a > check for it. That check adds overhead in the non-pre-sorted case to > improve the pre-sorted case, and nobody's ever argued for removing it > that I can recall.
I think that there has been a fair amount of skepticism - e.g., [1] But that's beside the point. What I mean is that I think that the intersection of those three things - pre-sorted input, with all differences after the first 8 bytes, and with a user requirement to sort using the column - is fairly rare in practice. It's not impossible, but it is fairly rare. If that was the only case that was actually regressed, even taking into account fmgr elision, I think that would be quite reasonable. It wouldn't be massively regressed either, and it's a case that's already very fast relative to other systems anyway, if you're lucky enough to get it. You'd better have exactly sorted tuples, though. It's been my observation that one slight difference can drastically alter the outcome [2]. [1] http://www.postgresql.org/message-id/18033.1361789...@sss.pgh.pa.us [2] http://www.postgresql.org/message-id/caeylb_w++uhrcwprzg9tybvf7sn-c1s9olbabvavpgdep2d...@mail.gmail.com -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
