Re: uuid type for postgres

Tom, you worded my thoughts much better than I did.

Bob, I too had heard that host-based UUIDs/GUIDs had issues with uniqueness.
I think Microsoft's implementation was hosed and they ended up eliminating
using the MAC completely.

I'll check out the code & get back.

On 9/6/05, Bob Ippolito <bob(at)redivi(dot)com> wrote:
>
>
> On Sep 6, 2005, at 3:06 PM, mark(at)mark(dot)mielke(dot)cc wrote:
>
> > On Tue, Sep 06, 2005 at 05:54:34PM -0400, Tom Lane wrote:
> >
> >> I don't see any "big opposition". People are simply questioning the
> >> idea whether it belongs in core PG. The reason we don't want to
> >> accept
> >> everything-and-the-kitchen-sink in core is that we have only limited
> >> manpower to maintain it. So you've got to justify that we should
> >> spend
> >> our effort here and not elsewhere. There's a fair amount of nearly
> >> ...
> >> been there awhile. So one of the questions that's going to be
> >> asked is
> >> how useful/popular it's really going to be.
> >>
> >
> > Sounds reasonable, and certainly no more than I expected. If Nathan
> > hadn't raised the issue, it probably would have been a few months
> > before I raised it myself.
> >
> >
> >> One thing that is raising my own level of concern quite a bit is the
> >> apparent portability issues. Code that isn't completely portable
> >> is a
> >> huge maintainability problem; in particular, stuff that requires
> >> system-dependent behavior used nowhere else in Postgres is a real
> >> pain.
> >> It sounds like the UUID code expects to be able to get at the
> >> machine's
> >> MAC address, which suggests serious issues in (a) relying on
> >> not-too-standard APIs, (b) possible protection issues (will an
> >> unprivileged process be able to get at the MAC address?), and (c)
> >> ill-defined behavior on machines with more or less than one MAC
> >> address.
> >> Not to mention that MAC addresses aren't so unique as all that.
> >>
> >
> > I'll try to prepare an answer for this. (I started to write a lot of
> > information - but is it unverified from memory, and perhaps should be
> > more authoritative before presented as truth)
>
> Some modern UUID implementations prefer /dev/urandom or similar to
> the time or MAC address unless you really beg them to give you a
> weaker UUID.
>
> You can take a look at the man page for the Theodore Y. Ts'o
> implementation that is in Darwin's Libc here:
> http://developer.apple.com/documentation/Darwin/Reference/ManPages/
> man3/uuid_generate.3.html
>
> Specifically:
>
> The uuid_generate function creates a new universally unique
> identifier
> (UUID). The uuid will be generated based on high-quality
> randomness
> from /dev/urandom, if available. If it is not
> available, then
> uuid_generate will use an alternative algorithm which uses
> the current
> time, the local ethernet MAC address (if available), and
> random data
> generated using a pseudo-random generator.
>
> The Apache Portable Runtime has a apr_os_uuid_get() that supports two
> flavors of UUID for unix (Linux/Mac OS X uuid_generate and FreeBSD's
> uuid_create, may be available elsewhere), and the UuidCreate API on
> Win32. apr-util's apr_uuid_get() will use apr_os_uuid_get() if
> available, and otherwise will default to a relatively weak mostly-
> timestamp-based UUID.
>
> It would probably be reasonable and easy to do what Apache does
> here. A platform UUID implementation, if present, is generally going
> to be better than anything included into PostgreSQL itself.
>
> -bob
>
>