Now where did THAT estimate come from? - Hugo Kornelis

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The SQL Server Query Optimizer makes its plan choices based on estimated rowcounts. If those estimates are wrong, the optimizer will very likely produce a poor plan. And there’s nothing you can do about it. Or is there?
In this session, you will learn exactly where these estimates come from. You will gain intimate knowledge of how statistics are built and maintained and how they are used to estimate row counts. But you will also learn how filters and joins influence those estimates.

Though the focus of this session is on understanding the causes of bad estimates, you will also learn ways to fix the problems and get better estimates – and hence, better performing queries

Hugo Kornelis is an established SQL Server Community expert. He writes, blogs, speaks, tech edits, and researches, focusing mostly on SQL Server performance and execution plans. He was the technical editor for the third edition of Grant Fritchey's "SQL Server Execution Plans". In 2018 he started a project to document all behaviour of SQL Server execution plans at his website, "the SQL Server Execution Plan Reference" (

When not working for the community, Hugo is busy at his day job: freelance database developer./consultant. Hugo has 20 years of experience on SQL Server in various roles. He has a strong database design background but has since specialized into query tuning and execution plans.