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Delayed Prefetch and Hidden Reads

Hugo Kornelis looks at when worlds collide:

So let’s check. The picture above shows, side by side, the properties of the Index Seek and the Key Lookup operator. They show that the Index Seek did 3 logical reads only, while Key Lookup did 650 logical reads. A clear indication where the majority of the work is done.

But wait. Aren’t we missing something?

The SET STATISTICS IO ON output indicates a total of 722 logical reads. The two screenshots above add up to 653 logical reads. Where are the other 69 logical reads?

Read on for the answer.

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