Jeff Davis wrote:
On Mon, 2011-05-02 at 11:10 -0400, Greg Smith wrote:
The position Merlin
has advocated here, that there should always be a natural key available
if you know the data well enough, may be true. But few people are good
enough designers to be sure they've made the decision correctly, and the
downsides of being wrong can be a long, painful conversion process.
Easier for most people to just eliminate the possibility of making a
mistake by using auto-generated surrogate keys, where the primary
problem you'll run into is merely using more space/resources than you
might otherwise need to have. It minimizes the worst-case--mistake make
in the model, expensive re-design--by adding overhead that makes the
average case more expensive.
Once you really try to define "natural" and "surrogate" keys, I think a
lot of the arguments disappear. I wrote about this a few years back:
http://thoughts.j-davis.com/2007/12/11/terminology-confusion/
In particular, I think you are falsely assuming that a natural key must
be generated from an outside source (or some source outside of your
control), and is therefore not reliably unique.
You can generate your own keys, and if you hand them out to customers
and include them on paperwork, they are now a part of the reality that
your database models -- and therefore become natural keys. Invoice
numbers, driver's license numbers, etc., are all natural keys, because
they are known about, and used, in reality. Usernames are, too, the only
difference is that you let the user choose it.
In contrast, a pointer or a UUID typically does not represent reality,
because no humans ever see it and no computer systems outside yours know
about it. So, it's merely an implementation detail and should not be a
part of the model.
Regards,
Jeff Davis
My wife works (at the sql level) with shall we say "records about
people". Real records, real people. Somewhere around 2 million unique
individuals, several million source records. They don't all have ssn,
they don't all have a drivers license. They don't all have an address,
many have several addresses (especially over time) and separate people
have at one time or another lived at the same address. You would be
surprise how many "bob smith"s where born on the same day. But then
they weren't all born in a hospital etc etc etc. A person may present
on any of a birth record, a death record, a hospital record, a drivers
license, a medical registry, a marriage record and so on. There simply
is no natural key for a human. We won't even worry about the
non-uniqueness of ssn. And please don't get her started on twins. :)
I can only imagine that other equally complex entities are just as
slippery when it comes time to pinpoint the natural key.
rjs
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