Hi Rhino,
Excellent question. Felt as you do, initially.
Here's what changed my mind.
Integer keys are fast. And small. Hence, they take very little RAM space.
They are contiguous. A missing PK is easy to find. There's a gap in the
number sequence.
Can't do this with the part description. No way to tell if a record is
missing.
Example: The system gets hung up or crashes and a reboot is needed.
How to test the integrity of the parts table. I.e., anything missing? Check
the PK for
continuity is a good place to start. With a timestamp I would even know the
date
where the file got truncated. Example. It's Dec 20th. The highest date in
the file is
Dec 1st at rec# 1203023. That's where the analysis would begin. Other files
that
didn't get truncated but have the related key # in them would tip me off as
to how
much is missing. Like an order file.
Speed. Especially where related files are concerned. Foreign keys. Links on
integer
fields are faster, smaller and more efficient. Keys remain smaller and
faster.
Activity testing: Let's say I do some statistical testing. Like how many
new parts
per month on average. Easy to do with the integer PK. Even easier if it has
a timestamp.
Then if the average suddenly drops or increases I would want to know why. Or
modify
my DB tables or coding. Note that the timestamp does not have to be in your
example
table. It could be in an insert/update table that just tracks what has been
added or updated
by PK, timestamp, activity type and updatedbyuserID.
So, there's 2 cents worth.
Wondering how relevant this is?
HTH,
Ken Wagner
----- Original Message -----
From: "Rhino" <[EMAIL PROTECTED]>
To: "mysql" <mysql@lists.mysql.com>
Sent: Wednesday, December 21, 2005 2:54 PM
Subject: Reason for Auto-increment primary keys?
One technique that I see a lot on this mailing list is people putting
auto-incremented integer primary keys on their tables.
Maybe I'm just "old school" but I've always thought that you should choose
a primary key based on data that is actually in the table whenever
possible, rather than generating a new value out of thin air.
The only exception that comes to mind is things like ID numbers; for
example, it is better to use an internally-generated integer for an
employee number than it is to use an employee's name. Even the combination
of first name and last name is not necessarily unique - I could cite a
real life example -and, of course, people can change their names. That
makes names less desireable than a generated value when you are trying to
uniquely indentify such entities. In such a case, a nice, reasonable short
integer is easier.
I just found this rather good definition of primary keys at
http://www.utexas.edu/its/windows/database/datamodeling/dm/keys.html. The
relevant bit says that a primary key must have:
- a non-null value for each instance of the entity
- a value that is unique for each instance of an entity
- a value that must not change or become null during the life of the each
instance of the entity
That article makes the same basic remarks about name vs. ID but makes the
point that it is more commonly the case that table designers will use
something like a social security number - an _externally_ generated
number - to distinguish between employees rather than an
internally-generated number.
But the trend in this mailing list is toward using generated values as
primary keys in virtually EVERY table, even when good primary keys can be
found in the (non-generated) data already existing in the table.
Now, I haven't done anything remotely resembling a quantified analysis so
maybe I'm wildly exaggerating this trend. But I do seem to recall a lot of
table descriptions with auto-generated keys and I don't think they were
all a name vs. ID scenario....
Has anyone else noticed a similar trend?
If this trend is real, it doesn't seem like a very good trend to me. For
example, if you were keeping track of parts in a warehouse, why would
anyone make a table that looked like this:
ID (autogenerated PK) PART_NO PART_DESCRIPTION
1 A01 Widget
2 B03 Grapple Grommet
3 A02 Snow Shovel
4 D11 Whisk
5 C04 Duct Tape
when this table is simpler:
PART_NO (PK) PART_DESCRIPTION
A01 Widget
B03 Grapple Grommet
A02 Snow Shovel
D11 Whisk
C04 Duct Tape
Would anyone care to convince me that the first version of the table is
"better" than the second version in some way?
I just want to be sure that no one has come along with some new and
compelling reason to autogenerate keys when perfectly good keys can be
found within the data already. I don't mind being "old school" but I don't
want to be "out to lunch" :-)
Rhino
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