Friday, December 5, 2008

Amateurism and SQL Server

The Marketing department drives everything at Microsoft, especially product design and development. A while back this group decided that SQL Server was an "administration-free" database. Their premise is that my grandmother should be able to design, construct and manage an enterprise data platform without having to bother about learning much of anything.

For example, the table creation wizard allows a completely unqualified person to actually create a table. It defaults to creating a primary key on an identity column using a clustered index.
This does insure that the table 1) has a PK 2) has a clustered index, and 3) the clustered index isn't going to cause page splitting, even though the table creator has little idea what any of this means. This is a good thing for someone with a small startup that needs a database and can't afford professional help. However, I see it in large corporations done by people who are considered professionals.

I have had discussions with many of them and found that because the wizard's default behavior, They believed that a primary key HAD to be clustered. Also because of the wizard's defaults, they have confused an identity with a real primary key. When I ask them what other column or group of columns is unique in the table, they don't know what I'm asking.

While helpful for an amatuer application, these defaults virtually assure that the database will not scale as a company grows. The placement of the clustered index is critical to good performance. Placing it on an identity in every case is a big mistake.

A nonclustered index will handle an identity lookup as well as a clustered index, so why waste the only clustered index on something that can be done as well by a nonclustered index? Clustered indexes can do certain things much better than nonclustered indexes, like range searches returning many records.

It only makes sense to place the clustered index on a column where it will make a difference. However, deciding which column to place it on requires knowledge and creative thought of which the wizard is not capable and apparently many db designers are not either. It is not that they don't know how to handle the issue: it is that they don't even know there is an issue.

Next post, why an identity is not a primary key.

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