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Understanding Estimation Variance

Aaron Bertrand discusses drift between estimates and actuals in execution plans:

Now, inaccurate estimates won’t always be a problem, but it can cause issues with inefficient plan choices at the two extremes. A single plan might not be optimal when the chosen range will yield a very small or very large percentage of the table or index, and this can get very hard for SQL Server to predict when the data distribution is uneven. Joseph Sack outlined the more typical things bad estimates can affect in his post, “Ten Common Threats to Execution Plan Quality:”

Read the whole thing.