Benefits of an "Autonumber" Synthetic Key

I've come to creating all my tables using an auto-number, integer primary key called "ID".

In other words:

CREATE TABLE Person
(
    ID INT IDENTITY(1,1) PRIMARY KEY,
    ...
)

In another post, I talked about the good characteristics of a synthentic, non-meaningful key. In summary, meaningful keys are likely to need to change at some point, which can cause a problem because:

• all foreign keys will need updated
• some foreign keys may not be easily updatable, for instance older records that are archived, or off-line

So if you are going to synthesize, or completely invent, a field to be the primary key, what do you want out of that field?

In order to guarantee Entity Integrity, and satisfy the Guaranteed Access Rule (GAR), all you really need is that a key field be UNIQUE and NOT NULL. (The GAR requires that every value in a database be uniquely addressable. This is accomplished by tablename.fieldname WHERE primarykey=x.)

Another necessity is to have enough unique values for the table in question. Remember that data can live a long time (years, decades). Ask this question: How many records might be inserted in this table over the next ten years? That might seem hard, but you can start with how many per day, and do the math. Also consider that your traffic may increase over time, especially if your system is useful.

What else would be nice?

First, the field value for a record should not change. As pointed out above, a changing value in a primary key field will require cascading updates to all foreign keys. This can be difficult to accomplish, especially as data becomes older, and perhaps not online.

Second, the field should be small. In general, using the number of bytes necessary, but no more, keeps your records concise, and improves the number of records that fit on a page. In other words, when the database engine has to do an expensive disk IO to get a page, you'd like to get as many records on that page as possible. You are paying a disk IO, you'd like to get as many records as possible. Would you rather get 10 pencils for a dollar or 12?

Third, it might be nice to arrange it so that new records enter in an orderly fashion. This is where an "AutoNumber" field is helpful (IDENTITY in MS SQL Server).

Here's a layman's example. Think of a line of cars at a tollbooth. Usually, cars arrive at the tollbooth, and go to the end of the line. There's no question about where they go, and they don't affect any other cars in the line. But Suppose that as cars arrive at the tollbooth, that they must be arranged in license plate order. First, it must be decided where they should be in the line. Then which other cars have to move, then how to move them, etc. That's lot of work, and will take a lot of time.

In the database setting it is also convenient to have new records get placed "at the end". If your table uses an ascending "AutoNumber" field as the primary key, new records will get placed at the end. Furthermore, the database won't even have to ask the quesion "where does this record belong?", it just puts it at the end. No previously inserted records need to be relocated.

And the toll booth analogy goes further. Cars go through the tollbooth, leaving the line. The "oldest" cars go through the tollbooth first. Suppose we were making cars line up by licence plate number. Then we said, "The car that has been here longest gets to go through the tollbooth first!" We'd have to figure out which one was oldest, then make other cars move, etc. That's crazy!

In a database, records can leave the transactional table as they are archived. These are typically, the "oldest" records. If you are using an AutoNumber primary key, all the records that need to get archived are together, at the beginning of the list.

Finally, using an AutoNumber tends to keep "hot" records in RAM, rather on disk, improving performance. In a transactional database the newest records tend to accessed more often. The older records tend to get accessed less often. As an example, suppose I place an order at an e-tailer. I'm much more likely to change my mind and modify the order within 10 minutes of placing the order than after a week.

Using an AutoNumber means that the newest records are all together, on the same data pages. The database engine is smart enough to keep frequently accessed data pages in RAM to avoid the expensive disk access charges. As an example, I've noticed that my dentist office pulls all the files for the current day, and keeps them at the check in desk. So they are convenient and quick to access. Yesterday's files are filed in the back, because we won't need them for awhile.

To summarize, Autonumber fields are helpful because:

• they get inserted in order, without affecting other records
• they get archived in order, without affecting other records
• they help the database keep current records in RAM

Of course, there are exceptions to every rule. The advice above would apply to transactional systems with a variety of operations (INSERT, UPDATE, DELETE, SELECT).

Benefits of a Synthetic Primary Key

Why make up a primary key, when there's a perfectly good primary key already available?

Great question. I love it. It shows thought and a recognition that there might be a consequence to the choice. 

For a field to be a primary key, all we need is to make sure that every record has a value in that field, and that there are no duplicates, i.e., UNIQUE and NOT NULL.

There is no requirement that a primary key field 

1. have any meaning in the real world
2. be mnemonic (remind you of something)
3. be printed out on anything
4. be derivable from other information
5. be available or of interest to anyone other than a database professional

Each of the items above is helpful and useful, but is not strictly needed from a primary key. In fact, some of these helpful characteristics can actually cause practical problems in fields that are used as a primary key.

(There are significant performance implications for this choice, but I'm going to speak to more practical reasons why a completely made-up primary key has benefits.)

Consider the following table called Person, with the following fields:

• firstName
• lastName
• SSN
• username

For a field to be usable as a primary key, it must minimally be NOT NULL, and UNIQUE. This table, potentially, has fields that we might consider enforcing these requirements on:

• SSN
• username

Let's look first at SSN. This would normally be CHAR(9). As the database designer, here are some things to consider about SSN:

• it means something in other systems
• it requires that the Person exist in another system
• it's not a strictly necessary attribute of a Person entity, i.e., People exist that don't have SSNs
• there's no reasonable validity check on SSN values

Let's assume that our database designer has decided to use SSN as the primary key for the Person table. Somehow, the value '112456678' is listed as John Smith's SSN. How do we know that this is really John Smith's SSN? Suppose it isn't? John contacts us and states that the last character that is '8' should be '3'. So we make the change, and realize that all foreign key references must also be updated. OK, we cascade updates (manually or automatically) through all the foreign key references. What about last month's data from last month that is in the data warehouse, and not "cascadable"? What about the archive data from last year that's not online?

It's not that it's impossible to make all those changes, it's just expensive. Or, we can make the economically sensible decision to not update all the cascading data. Effectively, in our system, John Smith will have become two completely different people, because his primary key will have changed. 

SSN is a great example of what I call an External Key. It's a value that's useful in identifying a person in someone else's system; in this case, the IRS. We have to give it to the IRS when we want to reference a specific Person in their database. However, because it's really the IRS's data, we cannot know, definitively, that it is accurate. 

If it was our data, we could definitively verify that it was correct. Or, we could just decide that it is correct. For example, supposer that we were the IRS, and SSN was our data, and we found that John Smith's last SSN character was '8', but he thought it should be '3'. As long as '112456678' was unique and not null, we could just leave it as is.

So how about using username as a primary key? Let's assume that username is a value that we have complete control over - that we have some method for determining it, and no other system uses it. 

Since we have complete control over it, how should we do it? What makes a good username? It's common to use some scheme like 'SmithJ', i.e., last name, concatenated with the first letter of the first name. People like that, mainly because of certain characteristics:

1. it has some meaning in the real world - someone's lastname...
2. it's mnemonic - it helps us remember which person goes with the username
3. it would be helpful useful information to print out on things
4. it's derivable from other data
5. it's useful for people in the real world (not db professionals)

However, all the things that make it useful also make it problematic as a primary key. We still have the problems that SSN had:

• it means something in other systems (in this case, the social system - we call people by their name)
• there's no reasonable validity check on names

What happens when someone changes their name? All the same things that happen when we have a mistake in a SSN. We don't control names - people control their own names.

So let's create usernames that don't mean something in the real world! Then it would make a good Primary Key!

Then we lose all the good attributes of a username.

My default recommendation is to have a primary key that is for the database professionals, and has no meaning to anyone else. Something completely non-meaningful; a completely Synthetic Key, that means nothing to anyone else but a db professional.

Why? 

Data Integrity. When a key changes, we lose Referential Integrity. If we cannot control a key value, we cannot guarantee data integrity.

Database professionals need dependable (unchanging) primary keys so that they can be used to relate data. 

Of course, we also want primary keys that will help with performance, but that's for another post...