The reason is that using OR in a query will often cause the Query Optimizer to abandon use of index seeks and revert to scans. If you look at the execution plans for your two queries, you'll most likely see scans where you are using the OR and seeks where you are using the UNION . Without seeing your query it's not really possible to give you any ideas on how you might be able to restructure the OR condition. But you may find that inserting the rows into a temporary table and joining on to it may yield a positive result.

Also, it is generally best to use UNION ALL rather than UNION if you want all results, as you remove the cost of row-matching.

There is currently no way in SQL Server to force a UNION execution plan if no UNION statement was used. If the only difference between the two parts is the WHERE clause, create a view with the complex query. The UNION query then becomes very simple:

SELECT * FROM dbo.MyView WHERE <cond1>
UNION ALL
SELECT * FROM dbo.MyView WHERE <cond2>

It is important to use UNION ALL in this context when ever possible. If you just use UNION SQL Server has to filter out duplicate rows, which requires an expensive sort operation in most cases.