On Sun, Jun 30, 2013 at 8:30 AM, Atri Sharma <atri.j...@gmail.com> wrote: > I have been reading the recent discussion and was researching a bit, and I > think that we should really go with the idea of randomising the input data(if > it is not completely presorted), to ensure that we do not get quadratic > complexity.
That doesn't ensure any such thing. It just makes it less likely. But we have existing guards that also make that unlikely, so I'm not sure what we'd be gaining. > One easy way to do that could be to take a sample of the data set, and take a > pivot out of it. Still a better way could be to take multiple samples which > are spread of the data set, select a value from each of them, and then take a > cumulative pivot(median,maybe). We pretty much do that already. > This shouldn't be too complex, and should give us a fixed nlogn complexity > even for wild data sets, without affecting existing normal data sets that are > present in every day transactions. I even believe that those data sets will > also benefit from the above optimisation. The only method of selecting a pivot for quicksort that obtain O(n lg n) run time with 100% certainty is have a magical oracle inside the computer that tells you in fixed time and with perfect accuracy which pivot you should select. If you want to get a useful response to your emails, consider including a statement of what you think the problem is and why you think your proposed changes will help. Consider offering a test case that performs badly and an analysis of the reason why. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers