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| Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? [message #178389 is a reply to message #178386] |
Tue, 12 June 2012 19:26   |
Peter H. Coffin
Messages: 245
Registered: September 2010
Karma: 0
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Senior Member |
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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.
--
48. I will treat any beast which I control through magic or technology
with respect and kindness. Thus if the control is ever broken, it
will not immediately come after me for revenge.
--Peter Anspach's list of things to do as an Evil Overlord
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| 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
Karma: 0
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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!
--
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex(at)attglobal(dot)net
==================
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| Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? [message #178397 is a reply to message #178396] |
Wed, 13 June 2012 11:40 |
Goran
Messages: 38
Registered: January 2011
Karma: 0
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Member |
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On 13.6.2012 13:09, Captain Paralytic wrote:
> On Jun 12, 7:47 pm, "ku...@pobox.com" <ku...@pobox.com> wrote:
>>> No, hashes can never guaranteed to be unique.
>>
>> I opted to use
>>
>> md5(uniqid());
>>
>> to create a unique key.
>
> Why bother hashing it, why not just use the uniqid() value directly?
uniqid() is not truly unique either:D Only truly unique value is the one
generated by DB (if we are talking about DB data).
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| Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? [message #178457 is a reply to message #178397] |
Tue, 19 June 2012 10:32 |
Captain Paralytic
Messages: 204
Registered: September 2010
Karma: 0
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Senior Member |
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On Jun 13, 12:40 pm, Goran <go...@nospam.com> wrote:
> On 13.6.2012 13:09, Captain Paralytic wrote:
>
>> On Jun 12, 7:47 pm, "ku...@pobox.com" <ku...@pobox.com> wrote:
>>>> No, hashes can never guaranteed to be unique.
>
>>> I opted to use
>
>>> md5(uniqid());
>
>>> to create a unique key.
>
>> Why bother hashing it, why not just use the uniqid() value directly?
>
> uniqid() is not truly unique either:D Only truly unique value is the one
> generated by DB (if we are talking about DB data).
Err I know that! but hashing it actually REDUCES it's uniqueness.
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