When using a built-in function in your query, the query may not use the indexes on the table. For example, the following query needs to do a full scan of the table: 

USE lahmansbaseballdb;
SELECT UPPER(playerid) as playeridupper, playerid
FROM allstarfull
WHERE upper(playerid) = 'AARONHA01';

The following screenshot shows the execution plan of the previous query: 

The following query can seek the rows that are needed: 

USE lahmansbaseballdb;
SELECT playerid FROM allstarfull
WHERE playerid = 'aaronha01';

The following screenshot shows the execution plan of the previous query: 

You can see that the query that doesn't use a built-in function in the WHERE clause performs much better than the one that does. 

Reading and running execution plans are covered in more detail in Chapter 6, Querying a Single Table, and in Chapter 7, Querying Multiple Tables.