Question of the Day

Query Plan Regressions --

For the Question of the day, I am going to go deep, but try to be more clear, as I feel like I didn't give enough info last time, leading folks to guess the wrong answer... :) For today's question:  You’re troubleshooting a performance issue on a critical stored procedure. You notice that a previously efficient query now performs a full table scan instead of an index seek. Upon investigating, you find that an NVARCHAR parameter is being compared to a VARCHAR column in the WHERE clause. What is the most likely cause of the query plan regression?