Uniqueidentifier vs. IDENTITY vs. Material Code --which is the best choice for primary key?

Question

Which one is the best choice for primary key in SQL Server?
There are some example code:

Uniqueidentifiers

e.g.

CREATE TABLE new_employees    (employeeId   UNIQUEIDENTIFIER      DEFAULT NEWID(),    fname      VARCHAR(20) ) GO INSERT INTO new_employees(fname) VALUES ('Karin') GO 

Identity columns

e.g.

 CREATE TABLE new_employees  (   employeeId int IDENTITY(1,1),   fname varchar (20)  );   INSERT new_employees     (fname)  VALUES     ('Karin'); 

[Material Code](or Business Code,which identity of a material. e.g. customer identifier)

e.g.

CREATE TABLE new_employees(     [ClientId] [varchar](20) NOT NULL,     [fName] [varchar](20) NULL        )   INSERT new_employees     (ClientID, fname)  VALUES     ('C0101000001',--customer identifier，e.g.'C0101000001' a user-defined code.      'Karin'); 

Please give me some advices for choosing the primary key from the three type identity columns,or other choices.

Thanks!

Answer 1

GUID may seem to be a natural choice for your primary key - and if you really must, you could probably argue to use it for the PRIMARY KEY of the table. What I'd strongly recommend not to do is use the GUID column as the clustering key, which SQL Server does by default, unless you specifically tell it not to.

You really need to keep two issues apart:

1. the primary key is a logical construct - one of the candidate keys that uniquely and reliably identifies every row in your table. This can be anything, really - an INT, a GUID, a string - pick what makes most sense for your scenario.

2. the clustering key (the column or columns that define the "clustered index" on the table) - this is a physical storage-related thing, and here, a small, stable, ever-increasing data type is your best pick - INT or BIGINT as your default option.

By default, the primary key on a SQL Server table is also used as the clustering key - but that doesn't need to be that way! I've personally seen massive performance gains when breaking up the previous GUID-based primary / clustered key into two separate keys - the primary (logical) key on the GUID, and the clustering (ordering) key on a separate INT IDENTITY(1,1) column.

As Kimberly Tripp - the Queen of Indexing - and others have stated a great many times - a GUID as the clustering key isn't optimal, since due to its randomness, it will lead to massive page and index fragmentation and to generally bad performance.

Yes, I know - there's newsequentialid() in SQL Server 2005 and up - but even that is not truly and fully sequential and thus also suffers from the same problems as the GUID - just a bit less prominently so.

Then there's another issue to consider: the clustering key on a table will be added to each and every entry on each and every non-clustered index on your table as well - thus you really want to make sure it's as small as possible. Typically, an INT with 2+ billion rows should be sufficient for the vast majority of tables - and compared to a GUID as the clustering key, you can save yourself hundreds of megabytes of storage on disk and in server memory.

Quick calculation - using INT vs. GUID as primary and clustering key:

• Base Table with 1'000'000 rows (3.8 MB vs. 15.26 MB)
• 6 nonclustered indexes (22.89 MB vs. 91.55 MB)

TOTAL: 25 MB vs. 106 MB - and that's just on a single table!

Some more food for thought - excellent stuff by Kimberly Tripp - read it, read it again, digest it! It's the SQL Server indexing gospel, really.

• GUIDs as PRIMARY KEY and/or clustered key
• The clustered index debate continues
• Ever-increasing clustering key - the Clustered Index Debate..........again!
• Disk space is cheap - that's not the point!

Unless you have a very good reason, I would argue to use a INT IDENTITY for almost every "real" data table as the default for their primary key - it's unique, it's stable (never changes), it's narrow, it's ever increasing - all the good properties that you want to have in a clustering key for fast and reliable performance of your SQL Server tables!

If you have some "natural" key value that also has all those properties, then you might also use that instead of a surrogate key. But two variable-length strings of max. 20 chars each do not meet those requirements in my opinion.