Sunday, April 3, 2016

SQL Server 2014 - Cardinality Estimator - Part 1

It has been a while since I blogged. Just moved to few other mediums like speaking at user group and SQL Saturday events in the last few months and it has kept me away from blogging for a while. Now back to blogging again. Hope to do a better balancing act :)

Starting from where I left, which was on Statistics and Histograms, I now move on to Cardinality Estimator, one of the major changes of SQL Server 2014

What is Cardinality Estimator?

Cardinality Estimator is responsible for providing the estimates on number of rows returned for each operator. If you have observed a query plan before, the estimated row count is provided by the cardinality estimator. What happens behind the scene is

  1. Query optimizer instructs cardinality estimator (CE) to find the estimated rows for each operation ( read a table, filter, join etc )
  2.  Cardinality estimator checks statistics and histogram to find the number of rows returned
  3.  Query optimizer based on the inputs given by CE, picks the right operator and subsequently the right plan

Obviously, CE plays a key role in getting right plan. Incorrect estimates would lead to poor plan and spiral out into poor performance.

CE has remained unchanged since SQL Server 7.0 and SQL Server 2014 brought about changes on the way estimates are made and on the algorithm used in making estimates. 

Run the following query with actual plan ( Query ->Include Actual Execution Plan ) on "Adventureworks2012". 

WHERE ProductID < 100

Right click on the select operator and click on properties as shown below

Observe the property "CardinalityEstimatonModelVersion". Value equal to 70 indicates it is using Old CE.

Query uses old CE as the compatibility level of "Adventureworks2012" database is 110 (SQL 2012). Switch the compatibility level to 2014 or higher,
rerun the query and you will notice the "CardinalityEstimatorModelVersion" to be 120, which indicates it uses the new cardinality estimator.

In a series of posts ( expect around 10 ), I would be covering the various changes in new cardinality estimator.

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