I'm having a performance problem when I try to use datetime values in a query's WHERE clause. Performance is fine when I use a literal string that represents the datetime value:

SELECT * FROM BigTable WHERE 
TargetDate > '2003-03-01'

But the query slows down when I reference a variable:

SELECT * FROM BigTable WHERE 
TargetDate > @TestDate

Why is the second query much slower than the first?