A new cardinality estimator

As mentioned in the introduction, SQL Server 2014 introduced a new cardinality estimator, and in this and later versions, the old cardinality estimator is still available. This section explains what a cardinality estimator is, why a new cardinality estimator was built, and how to enable the new and the old cardinality estimators.

The cardinality estimator is the component of the query processor whose job is to estimate the number of rows returned by relational operations in a query. This information, along with some other data, is used by the Query Optimizer to select an efficient execution plan. Cardinality estimation is inherently inexact because it is a mathematical model that relies on statistical information. It is also based on several assumptions that, although not documented, have been known over the years – some of them include the uniformity, independence, containment, and inclusion assumptions. The following is a brief description of...