Re: Reformatting GUIDs

Charles Hooper a écrit :

> Vladimir M. Zakharychev wrote:
> > Charles Hooper wrote:
> > > I wonder if you can look at this from a different direction. What if
> > > you use the random number generating capabilities to generate each of
> > > the hex numbers between the dashes?
> > >
> > > For example, the second section of the GUID contains a four character
> > > hex number. If you have Oracle provide a hex number between hex 1000
> > > and hex FFFF, that would be sufficient to satisfy that portion of the
> > > GUID. If you pre-convert the hex numbers to their decimal equivalents,
> > > you will save Oracle a little work, but it will be more difficult to
> > > see what is happening.

[...]

> >
> > The only problem with this approach as I see it is that use of
> > DBMS_RANDOM will probably have adverse effect on "GU" part of the
> > "GUID" (GU = Globally Unique.) SYS_GUID() guarantees that all returned
> > values are globally unique, because afaik it uses system timestamp and
> > unique host information for GUID generation, among other things.
> > DBMS_RANDOM doesn't guarantee uniqueness of returned values, only
> > randomness.
> >
>
> I am not disagreeing with you, but I believe that the random number
> generator in Oracle can be seeded with the system time. The reason
> that a GUID is thought to be unique is due to the range of possible
> values, essentially 15 * 16^31 (or if the first digit can also be a 0,
> 16^32, or 16 * 16^31).

Yes and no, Charles...

UUIDs are usually globally unique because they identify a single point in space-time - that is they are essentially a concatenation of system time and node MAC address.

You are correct in that a variant uses a random generator (which must be of cryptographic quality) , but note that even this variant has a well
defined internal structure, that is required for a true uuid.

> Note that there is nothing stopping the OP from calling sys_guid() five
> times, each time extracting a different sub-string portion of the value
> returned.

Well, I may be wrong but I have a feeling that the OP may generate the same identifier twice with this algorithm, if mischance wants that a sub-string of the guid system clock component overflows...

BTW, if you are interested, the RFC on the subject is here : http://www.ietf.org/rfc/rfc4122.txt

Have a nice day, Charles

• Raoul