Änderungen im Cardinality Estimator SQL Server 2014

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Cardinality Estimation in SQL Server 2014
Klaus Aschenbrenner
Microsoft Certified Master SQL Server 2008
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Twitter: @Aschenbrenner

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• CEO & Founder SQLpassion
• International Speaker, Blogger, Author
• SQL Server 2008 MCM
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• Twitter: @Aschenbrenner
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Agenda
• Introduction to CE
• CE in SQL Server 2014

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Agenda
• Introduction to CE
• CE in SQL Server 2014

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What is Cardinality Estimation?

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Predicates
• Filter Predicates
– WHERE
– HAVING
• Join Predicates
– JOIN Condition

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Selectivity
• Defines how many rows are returned
• A number between 0.0 and 1.0
– 0.0: High Selectivity (no records)
– 1.0: Low Selectivity (all records)

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Questions answered by CE
• Selectivity of
– Single Filter Predicates
– Multiple Filter Predicates
– Join Predicate between 2 tables
• How many distinct values do we expect
– GROUP BY
– DISTINCT

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Agenda
• Introduction to CE
• CE in SQL Server 2014

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CE in SQL Server 2014
• No changes since SQL Server 7.0
– Changes were controlled through Trace Flags
– No Plan-Quality Regressions!
• CE in SQL Server 2014 is the 1st major change
– Should improve workloads
– But you WILL see regressions!

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Enabling the new CE
• Database Compatibility Level 120
• Trace Flags
– 2312: uses the new CE
– 9481: uses the old CE
– QUERYTRACEON at the statement level

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Demo
Enabling the new CE

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CE Model Assumptions
• Independence
– Filter Predicates are not correlated
• Uniformity
– Values in a Histogram step are evenly distributed
• Inclusion
– Comparing to a constant yields in a match
• Containment
– If something is being searched for, it is assumed that it exists

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Improvements
• Multi-Column Predicates
– Conjunctions (AND)
– Disjunctions (OR)
• Ascending Key Column Problems
– Out of Range Estimations
• Join Algorithm Changes
• Troubleshooting through Extended Events

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Multi-Column Predicates - AND
• Conjunctions – old behavior
– Independence Assumption
– Selectivity of conjunctive predicates are multiplied
– Doesn’t reflect reality

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Multi-Column Predicates - AND
• Conjunctions – new behavior
– Predicates are sorted by their selectivity
– 4 most selective predicates are kept
– Predicates are softened by taking square roots
– Exponential back-off Algorithm

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Demo
Exploring Conjunctions

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Multi-Column Predicates - OR
• Disjunctions – old behavior
– Independence Assumption
– Individual selectivities are added, and the estimate of the conjunction
is subtracted
– Doesn’t reflect reality

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Multi-Column Predicates - OR
• Disjunctions – new behavior
– Independence Assumption
– Disjunctions are converted to Conjunctions
• “De Morgan’s Law”: http://en.wikipedia.org/wiki/De_Morgan's_laws
– Same Exponential back-off Algorithm is applied to the Conjunction

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Demo
Exploring Disjunctions

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Summary
• Introduction to CE
• CE in SQL Server 2014

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