> -----Original Message----- > From: Bruce Momjian [mailto:[EMAIL PROTECTED]] > Sent: Friday, June 21, 2002 9:52 AM > To: Tom Lane > Cc: Neil Conway; [EMAIL PROTECTED]; Dann Corbit; > [EMAIL PROTECTED] > Subject: Re: [HACKERS] What is wrong with hashed index usage? > > > Tom Lane wrote: > > Bruce Momjian <[EMAIL PROTECTED]> writes: > > > I remember three problems: build time, index size, and > concurrency > > > problems. I was wondering about the equal key case > myself, and assumed > > > hash may be a win there, but with the concurrency > problems, is that even > > > possible? > > > > Sure. Many-equal-keys are a problem for btree whether you have any > > concurrency or not. > > > > > OK, I have reworded it. Is that better? > > > > It's better, but you've still discarded the original's > explicit mention > > of concurrency problems. Why do you want to remove information? > > OK, concurrency added. How is that? > > > > > > How about an elog(NOTICE) for hash use? > > > > I don't think that's appropriate. > > I was thinking of this during CREATE INDEX ... hash: > > NOTICE: Hash index use is discouraged. See the CREATE INDEX > reference page for more information. > > Does anyone else like/dislike that?
I think it might be OK temporarily, to show that there is some work that needs done. When hashed indexes are fixed, the notice should be removed. I have not looked at the hash code. Here is a strategy (off the top of my head) that seems that it should work: Use Bob Jenkins' 64 bit generic hash from here (totally free for use and fast as blazes): http://burtleburtle.net/bob/hash/index.html Specifically: http://burtleburtle.net/bob/c/lookup8.c and routine: hash( k, length, level) Now, with a 64 bit hash, there is very tiny probability of a collision (but you could have duplicate data). The hash index would consist of nothing more than this: [long long hash=64 bit hash code][unsigned nmatches=count of matching hashes][array of {nmatches} pointers directly to the records with that hash] This is probably grotesqely oversimplified. But maybe it will spur an indea in the person who writes the indexing code. ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
