At the heart of SQL Server is the cost based optimizer.  Stop and think about that a minute, it
attempts to give the “best plan” based on the cost of the work undertaken.  How does it know the cost of the work before
its done the work ? This isn’t a conundrum, it doesn’t.  It estimates! 
How does it estimate ? That is statistics.

This will be a deep dive into how the optimizer makes its
decisions to give you a plan, the things that can go wrong and how you can have
influence over these choices.

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