Re: [HACKERS] Patch for UUID datatype (beta)

On Tue, Sep 19, 2006 at 09:51:23AM -0400, mark@mark.mielke.cc wrote:
> On Tue, Sep 19, 2006 at 08:20:13AM -0500, Jim C. Nasby wrote:
> > On Mon, Sep 18, 2006 at 07:45:07PM -0400, mark@mark.mielke.cc wrote:
> > > I would not use a 100% random number generator for a UUID value as was
> > > suggested. I prefer inserting the MAC address and the time, to at
> > > least allow me to control if a collision is possible. This is not easy
> > > to do using a few lines of C code. I'd rather have a UUID type in core
> > > with no generation routine, than no UUID type in core because the code
> > > is too complicated to maintain, or not portable enough.
> > As others have mentioned, using MAC address doesn't remove the
> > possibility of a collision.
>
> It does, as I control the MAC address. I can choose not to overwrite it.
> I can choose to ensure that any cases where it is overwritten, it is
> overwritten with a unique value. Random number does not provide this
> level of control.
>
> > Maybe a good compromise that would allow a generator function to go into
> > the backend would be to combine the current time with a random number.
> > That will ensure that you won't get a dupe, so long as your clock never
> > runs backwards.
>
> Which standard UUID generation function would you be thinking of?
> Inventing a new one doesn't seem sensible. I'll have to read over the
> versions again...

I don't think it exists, but I don't see how that's an issue. Let's look
at an extreme case: take the amount of random entropy used for the
random-only generation method. Append that to the current time in UTC,
and hash it. Thanks to the time component, you've now greatly reduced
the odds of a duplicate, probably by many orders of magnitude.

Ultimately, I'm OK with a generator that's only in contrib, provided
that there's at least one that will work on all OSes.
--
Jim Nasby                                    jimn@enterprisedb.com
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)