On Friday, January 21, 2011 09:23:47 am Jerry Schwartz wrote: > > [JS] A UUID (what Microsoft calls a GUID) is based in part on the MAC > address of the generating device. Since MAC addresses are supposed to be > unique across the known universe, so should a UUID. >
Not entirely true - and even when true your conclusion doesn't follow. I'll be the first to admit that we are straining at gnats, here. Generating a UUID does not require a machine to have an Ethernet card. The presence of a Mac address is, therefore, not guaranteed. Even if the machine does have an ethernet card, the definition of the UUID suggests - but does not require - that it be used (I believe this is discussed in another reply). Even if it is used, there are still a (admittedly large) portion of it that is created by a random number generation. Assuming a reasonably good generator, it is still possible for a collision on a single machine. The question then becomes, how much of a problem is that? The odds are that I'll be LONG gone before that happens. But it could happen tomorrow. If it's just a matter of "Oops, lets do a rollback and go again", who cares? If it's a life sustaining mission critical problem where an atomic war starts if the transaction fails ... you might want to at least think about another key. Personally, I wouldn't lose sleep over it. Another reasonable - and faster - source of a unique key would be to have a human set unique server id on each machine generating the key. In that case a two field primary key with the first being the server id and the second part being an auto_increment field should guarantee a unique key - at least until it wraps around. This, of course, assumes that the administrator that sets up the servers doesn't make a mistake with setting the server id. Which brings up the "how much of a problem would that be" question again. Bottom line ... exactly how paranoid are you? ---Michael -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql?unsub=arch...@jab.org
