You've probably heard the term statistics bandied about in discussions of SQL Server query performance. Statistics are meta-data that SQL Server maintains about index keys and, optionally, nonindexed column values. SQL Server uses statistics to determine whether using an index could speed up a query. In conjunction with indexes, statistics are the single most important source of data for helping the optimizer develop optimum execution plans. When statistics are missing or out-of-date, the optimizer's ability to formulate the best execution plan for a query is seriously impaired.
Let's cover a few basic statistics-related terms before we discuss statistics in more depth.
The cardinality of data refers to how many unique ...