Published on

March 1, 2008

The Impact of Function Use in SQL Server WHERE Clauses

When writing SQL statements, it is important to consider the performance implications of using functions in the WHERE clause. SQL Server’s ability to efficiently use indexes plays a crucial role in query performance. One common mistake is wrapping filtering columns inside functions, which can prevent the optimizer from utilizing indexes effectively.

Let’s consider an example. Suppose we have a table called CadencedEventCustomer with a nonclustered index on the FullName1 column. We want to retrieve records where the FullName1 is ‘Ed Jones’ and handle null values correctly. A common approach is to use the IsNull function:

Select * FROM CadencedEventCustomer WHERE isNull(FullName1,'') = 'Ed Jones'

However, by wrapping the FullName1 column in the IsNull function, the query optimizer cannot see the column and the index is not used. Instead, an index scan is performed, which means the entire table is searched from beginning to end. This can be extremely slow, especially for large tables.

To improve performance, we can rewrite the query without using the IsNull function:

Select * FROM CadencedEventCustomer WHERE FullName1 = 'Ed Jones'

Now, the optimizer can see the indexed column and perform an index seek, resulting in significantly faster query execution.

This behavior is not limited to the IsNull function. Other functions, such as Substring and DateDiff, can also hide columns from the optimizer and prevent index usage. For example:

WHERE SUBSTRING(MasterDealer.Name,4) = 'Ford'

This query hides the MasterDealer.Name column from the optimizer and negates the use of any index on the Name column. To improve performance, we can rewrite the query as:

WHERE MasterDealerName Like 'Ford%'

This provides the same result while allowing the optimizer to utilize the index.

Similarly, when working with dates, functions like DateDiff can hide columns and impact performance. For example:

WHERE DateDiff(mm,PlacedOnQueue,GetDate()) >= 30

To improve performance, we can rewrite the query as:

WHERE PlacedOnQueue < DateAdd(mm,-30,GetDate())

Again, this provides the same result while allowing the optimizer to utilize the index.

It is important to note that this behavior applies to all functions wrapping columns in the WHERE clause. Whenever possible, try to rewrite queries without using functions. By doing so, you can achieve better performance and optimize the use of indexes.

If you need assistance in rewriting queries or have any questions, feel free to reach out to us.

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