Question

I’ve heard that using an IN Clause can hurt performance because it doesn’t use Indexes properly. See example below:

SELECT ID, Name, Address 
FROM people
WHERE id IN (SELECT ParsedValue FROM UDF_ParseListToTable(@IDList))

Is it better to use the form below to get these results?

SELECT ID,Name,Address
FROM People as p
INNER JOIN UDF_ParseListToTable(@IDList) as ids
ON p.ID = ids.ParsedValue

Does this depend on which version of SQL Server you are using? If so which ones are affected?

Answer 1

Yes, assuming relatively large data sets.

It’s considered better to use EXISTS for large data sets. I follow this and have noticed improvements in my code execution time.

According to the article, it has to do with how the IN vs. EXISTS is internalized. Another article: http://weblogs.sqlteam.com/mladenp/archive/2007/05/18/60210.aspx