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Understanding Missing Index Impact

Erik Darling delves into the depths of missing indexes:

Breaking each of those down, the only one that has a concrete meaning is Uses, but that of course doesn’t mean that a query took a long time or is even terribly inefficient.

That leaves us with Average Query Cost, which is the sum of each operator’s estimated cost in the query plan, and Impact.

But where does Impact come from?

Read on to learn where, as well as why you shouldn’t blindly trust that number.