| Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? [message #178394 is a reply to message #178389] |
Tue, 12 June 2012 23:34   |
Jerry Stuckle
Messages: 2598
Registered: September 2010
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Senior Member |
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On 6/12/2012 3:26 PM, Peter H. Coffin wrote:
> On Tue, 12 Jun 2012 11:47:04 -0700 (PDT), kurtk(at)pobox(dot)com wrote:
>>> No, hashes can never guaranteed to be unique.
>>
>> I opted to use
>>
>> md5(uniqid());
>>
>> to create a unique key.
>
> Still not guaranteed to be unique. You've even got a defined 13-hexdigit
> space thing what you're hashing. Which means as soon as you've got more
> than 4,503,599,627,370,495 entries, you are GUARANTEED a collision. That
> seems like a lot, but you're down to a one in a billion shot at a
> collision when you've got a few million entries. And everybody knows,
> that one in a billion changes turn up nine times in ten.
>
Peter,
I haven't figured out the exact numbers, but I suspect it's much less
than one in a billion to have a 50-50 chance.
After all - it only takes 24 people to have a 50-50 chance two of them
have the same birthday.
Odds go down quite a bit more quickly than you think!
--
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Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex(at)attglobal(dot)net
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