Now Where Did THAT Estimate Come From?

2024

TL; DR

This session is a deep dive into cardinality estimation. It starts with the basics, covering statistics, predicates, and correlation. But we then move on to hardly documented advanced topics, such as aligning histograms to estimate the cardinality after a join.

Session Details

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 cause of bad estimates, you will also learn ways to fix the problems and get better estimates - and hence, better performing queries.

3 things you'll get out of this session

Hugo Kornelis's other proposed sessions for 2026

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Execution plans ... where do I start? - 2026

Execution plans explained - 2026

Execution Plans in Depth - 2026

Five stages of grief - internals of a hash spill - 2026

Here’s the execution plan … now what? - 2026

Normalization beyond Third Normal Form - 2026

Normalization beyond Third Normal Form, part 2 - 2026

Performance and execution plan improvements in SQL Server 2025 - Part 1 - 2026

Performance and execution plan improvements in SQL Server 2025 (part 2) - 2026

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