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From:Steve Meyers Date:October 23 2001 5:27pm
Subject:RE: Length limit of 500 on primary keys?
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> > 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"?
> 

In a previous message, I failed to mention one of the main reasons you would NOT want to
use a 500-character primary key.  MySQL uses a key buffer to keep as much index
information in memory as possible.  The longer the key, the less info it can keep in
memory, and the more often it will have to swap to disk.  If your key doesn't fit in the
key buffer, my tests have shown that there is a HUGE performance loss.

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