On Fri, Sep 12, 2014 at 2:58 PM, Peter Geoghegan <p...@heroku.com> wrote: > On Fri, Sep 12, 2014 at 11:38 AM, Robert Haas <robertmh...@gmail.com> wrote: >> Based on discussion thus far it seems that there's a possibility that >> the trade-off may be different for short strings vs. long strings. If >> the string is small enough to fit in the L1 CPU cache, then it may be >> that memcmp() followed by strcoll() is not much more expensive than >> strcoll(). That should be easy to figure out: write a standalone C >> program that creates a bunch of arbitrary, fairly-short strings, say >> 32 bytes, in a big array. > > While I think that's fair, the reason I didn't bother playing tricks > with only doing a (purely) opportunistic memcmp() when the string size > is under (say) CACHE_LINE_SIZE bytes is that in order for it to matter > you'd have to have a use case where the first CACHE_LINE_SIZE of bytes > matched, and the string just happened to be identical in length, but > also ultimately differed at least a good fraction of the time. That > seems like the kind of thing that it's okay to care less about. That > might have been regressed worse than what you've seen already. It's > narrow in a whole new dimension, though. The intersection of that > issue, and the issues exercised by Heikki's existing test case must be > exceedingly rare. > > I'm still confused about whether or not we're talking at cross > purposes here, Robert. Are you happy to consider this as a separate > and additional question to the question of what to do in an > abbreviated comparison tie-break?
I think I've said a few times now that I really want to get this additional data before forming an opinion. As a certain Mr. Doyle writes, "It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts." I can't say it any better than that. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
