List:General Discussion« Previous MessageNext Message »
From:Steve Meyers Date:October 23 2001 5:07pm
Subject:RE: Length limit of 500 on primary keys?
View as plain text  
> > 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.

Steve Meyers


Thread
Length limit of 500 on primary keys?Shankar Unni21 Oct
  • RE: Length limit of 500 on primary keys?Chris Bolt21 Oct
    • RE: Length limit of 500 on primary keys?Steve Meyers21 Oct
      • Re: Length limit of 500 on primary keys?Shankar Unni22 Oct
        • RE: Length limit of 500 on primary keys?Steve Meyers22 Oct
          • Re: Length limit of 500 on primary keys?Jeremy Zawodny23 Oct
            • RE: Length limit of 500 on primary keys?Steve Meyers23 Oct
              • RE: Length limit of 500 on primary keys?Steve Meyers23 Oct
                • Re: Length limit of 500 on primary keys?Shankar Unni24 Oct
              • Re: Length limit of 500 on primary keys?Bill Adams23 Oct
                • RE: Length limit of 500 on primary keys?Steve Meyers23 Oct
              • RE: Length limit of 500 on primary keys?Don Read23 Oct
            • Re: Length limit of 500 on primary keys?Dan Nelson24 Oct
Re: Length limit of 500 on primary keys?Heikki Tuuri23 Oct