Execution Plan Cost Estimates

SQL Server 2005, SQL Server 2008, SQL Server 2014, TSQL
It's been emphasized over and over that the costs of operations within an execution plan, and the estimated costs of the plan themselves are, in fact, estimates. But it goes further than that. The estimated values are based on statistics, or the lack thereof. Statistics themselves are also estimates. This means that the costs you're seeing are extrapolations based on extrapolations. So, you should just ignore those values and move on, right? Wrong. In order to understand how the optimizer is choosing to put together an execution plan for your query so that you can use that understanding to then make intelligent choices as to modifying the query or the structure of your database, you must use the values you have at hand. However, you must also understand where and…
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