Nested Loop Estimation

Kendra Little describes the nested loop operator in execution plans:

One of the first things we often learn when we’re looking at plans is that SQL Server uses estimates. And sometimes, those estimates are wrong. At first glance, this looks really wrong– it estimated 11.5 rows, and actually got 20,825 rows!

Read the whole thing to see how nested loop estimates which seem way off might not actually be.

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