The document discusses cardinality estimation (CE) improvements in SQL Server 2014. CE is used to estimate the number of rows returned by query predicates and joins. The new CE in SQL 2014 improves on handling multi-column predicates using an exponential back-off algorithm, fixes issues with ascending key columns, and allows troubleshooting through extended events. The changes should improve query performance but may also cause regressions. Administrators are advised to test workloads thoroughly with the new CE enabled.

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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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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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