Question

I have the SQL query:

SELECT ISNULL(t.column1, t.column2) as [result]
FROM t

I need to filter out data by [result] column. What is the best approach regarding performance from the two listed below:

WHERE ISNULL(t.column1, t.column2) = @filterValue

or:

WHERE t.column1 = @filterValue OR t.column2 = @filterValue

UPDATE: Sorry, I have forgotten to mention that the column2 is always null if the column1 is filled.

Answer 1

I have measured the performance of both queries on SQL Sever 2008.

And have got the following results:

Both approaches had almost the same Estimated Subtree Cost metric.
But the OR approach had more accurate value of the Estimated Number of Rows metric.

So the query optimizer will build more appropriate execution plan for the OR than for ISNULL approach.