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Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column?

Started by"kurtk@pobox.com" <kurtk@pobox.com>
First post2012-06-11 17:16 -0700
Last post2012-06-19 03:32 -0700
Articles 9 — 5 participants

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  Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? "kurtk@pobox.com" <kurtk@pobox.com> - 2012-06-11 17:16 -0700
    Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? Jerry Stuckle <jstucklex@attglobal.net> - 2012-06-11 20:24 -0400
      Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? "kurtk@pobox.com" <kurtk@pobox.com> - 2012-06-12 11:47 -0700
        Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? Jerry Stuckle <jstucklex@attglobal.net> - 2012-06-12 15:06 -0400
        Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? "Peter H. Coffin" <hellsop@ninehells.com> - 2012-06-12 14:26 -0500
          Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? Jerry Stuckle <jstucklex@attglobal.net> - 2012-06-12 19:34 -0400
        Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? Captain Paralytic <paul_lautman@yahoo.com> - 2012-06-13 04:09 -0700
          Re: Is spl_object_hash unique in the SQL sense? Can it be used as a   unique SQL db column? Goran <goran@nospam.com> - 2012-06-13 13:40 +0200
            Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column? Captain Paralytic <paul_lautman@yahoo.com> - 2012-06-19 03:32 -0700

#6295 — Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column?

From"kurtk@pobox.com" <kurtk@pobox.com>
Date2012-06-11 17:16 -0700
SubjectIs spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column?
Message-ID<b3a88b31-765b-4537-adfd-563c4734c66a@googlegroups.com>
Does anyone know definitely whether the results of spl_object_hash be safely used as a unique database column?

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#6296

FromJerry Stuckle <jstucklex@attglobal.net>
Date2012-06-11 20:24 -0400
Message-ID<jr627b$aqj$1@dont-email.me>
In reply to#6295
On 6/11/2012 8:16 PM, kurtk@pobox.com wrote:
> Does anyone know definitely whether the results of spl_object_hash be safely used as a unique database column?

No, hashes can never guaranteed to be unique.

-- 
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
==================

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#6308

From"kurtk@pobox.com" <kurtk@pobox.com>
Date2012-06-12 11:47 -0700
Message-ID<a3acc71e-a388-47ed-97c5-72f4d04c40e5@googlegroups.com>
In reply to#6296
> No, hashes can never guaranteed to be unique.

I opted to use

  md5(uniqid()); 

to create a unique key.

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#6310

FromJerry Stuckle <jstucklex@attglobal.net>
Date2012-06-12 15:06 -0400
Message-ID<jr840g$que$2@dont-email.me>
In reply to#6308
On 6/12/2012 2:47 PM, kurtk@pobox.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.  Hashes never are.  But if you've 
only got a few thousand items, the chances of a collision are slim.

This is why databases have sequence or auto-increment values.

-- 
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
==================

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#6311

From"Peter H. Coffin" <hellsop@ninehells.com>
Date2012-06-12 14:26 -0500
Message-ID<slrnjtf5vi.h39.hellsop@nibelheim.ninehells.com>
In reply to#6308
On Tue, 12 Jun 2012 11:47:04 -0700 (PDT), kurtk@pobox.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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#6316

FromJerry Stuckle <jstucklex@attglobal.net>
Date2012-06-12 19:34 -0400
Message-ID<jr8jmh$3cp$2@dont-email.me>
In reply to#6311
On 6/12/2012 3:26 PM, Peter H. Coffin wrote:
> On Tue, 12 Jun 2012 11:47:04 -0700 (PDT), kurtk@pobox.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@attglobal.net
==================

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#6319

FromCaptain Paralytic <paul_lautman@yahoo.com>
Date2012-06-13 04:09 -0700
Message-ID<bafa9abd-6775-4874-b575-a5c00afcd3d3@v9g2000vbc.googlegroups.com>
In reply to#6308
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?

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#6320 — Re: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column?

FromGoran <goran@nospam.com>
Date2012-06-13 13:40 +0200
SubjectRe: Is spl_object_hash unique in the SQL sense? Can it be used as a unique SQL db column?
Message-ID<jr9u7s$ln4$1@news.metronet.hr>
In reply to#6319
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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#6381

FromCaptain Paralytic <paul_lautman@yahoo.com>
Date2012-06-19 03:32 -0700
Message-ID<7e2e84b8-9c28-474e-b7c8-646e996bda85@n33g2000vbi.googlegroups.com>
In reply to#6320
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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