Hi! I changed now ha_innobase.h so that InnoDB allows keys up to 7000 bytes in length. The change will be in MySQL-3.23.44 and 4.0.1.
Regards, Heikki http://www.innodb.com >Steve Meyers wrote: > >> > > At a previous job, we tested a 32-bit hash function by running it >> > > against hundreds of thousands of unique URL's stored in our >> > > database. We found one collision. A 64-bit hash is billions of >> > > times better (4 billion, to be exact). >> > >> > Good to know. I wonder how many collisions I'd find if I ran it over >> > every URL listed in the directory www.yahoo.com. >> > >> > Which 64 bit hash function did you use? Invent your own, or something >> > "off the shelf"? >> > >> >> We found a public domain one on the net see http://www.burtleburtle.net/bob/hash/evahash.html >for some sample code. It's only a 32-bit hash though. However, that same page >appears to have instructions for a 64-bit hash function as well, but I haven't tried >it at all. I'd be curious to know how many collisions you find hashing all the >URL's in yahoo's database :) I don't know how long that would take, but if you >do it I'd like to hear the results. >> >> Since the hash function takes a key and an initial value, you could try running >it with two different initial values and/or keys. This would give you effectively >a 128-bit hash, which you could store across two fields in MySQL. I'm guessing >that the 64-bit hash will probably be good enough though. > >I am not understanding why having a hash and the full url in the database would >not take care of the collisions. Even if you had 10 collisions for a 16 bit hash >(say), if your query was: >SELECT ... WHERE hash=thehashvalue AND url='theurl' you would get very fast lookups >on the hash and the url comparison would not add much to the query at that point. > You could even do a partial index on the url, e.g. "KEY( hash, url(200))". > >b. > > > --------------------------------------------------------------------- Before posting, please check: http://www.mysql.com/manual.php (the manual) http://lists.mysql.com/ (the list archive) To request this thread, e-mail <[EMAIL PROTECTED]> To unsubscribe, e-mail <[EMAIL PROTECTED]> Trouble unsubscribing? Try: http://lists.mysql.com/php/unsubscribe.php
