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From:Heikki Tuuri Date:October 23 2001 5:54pm
Subject:Re: Length limit of 500 on primary keys?
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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.
>
>
>


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