In my project I see that people writes different things with EXISTS logical operator (EXISTS (SELECT StarOrOne FROM...)). They are "*", "1" and even "TOP 1 1" or "TOP 1 *". The aim of this article to give a good explanation which option is the best.
2020-01-24 (first published: 2017-04-24)
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By Steve Jones
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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?
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