Re: Length limit of 500 on primary keys?

[Jeremy Zawodny]

On Mon, Oct 22, 2001 at 11:42:23AM -0600, Steve Meyers wrote:
> 
> If you use a good 64-bit hash, I doubt you'll run into any
> uniqueness problems.  MySQL will support that as a 64-bit BIGINT.
> You especially should not have any problems if you hash each column,
> then do the primary key across the four hashes.
> 
> I'm not sure why there is a limit, but I'm also not sure why anybody
> in their right mind would want a unique index that long :)

In Gemini, I think the limit is 2048 bytes (or so).  It was increased
to handle URLs (among other things).

> 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"?

Jeremy
-- 
Jeremy D. Zawodny, <jzawodn@stripped>
Technical Yahoo - Yahoo Finance
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