Understanding SQL Server statistics

In SQL Server, statistics are critical for the Query Optimizer to create efficient query plans. They are essentially metadata that contain statistical information about the distribution of values in one or more columns of a table or indexed view. Here’s a breakdown of what statistics in SQL Server entail:

• Components and Concepts: Statistics are binary large objects (BLOBs) that help the Query Optimizer estimate the cardinality, or number of rows, that a query result might return. This estimation is crucial for the Query Optimizer to decide on the best query plan, such as choosing an index seek over an index scan to improve performance¹.
• Histogram: A histogram is part of the statistics that measures the frequency of occurrence for each distinct value in a data set. SQL Server computes histograms on the first key column of the statistics object, which can be based on a full scan of the table or a sampled set of rows¹.
• When to Create and Update Statistics: SQL Server automatically generates necessary statistics for most queries. However, there are situations where you might need to create additional statistics or update existing ones to ensure optimal query performance. For instance, after significant changes to the data in a table, updating statistics can help maintain query performance¹.
• Auto Create Statistics: This is a feature that allows SQL Server to automatically create statistics for non-indexed columns when necessary during query execution, provided that the Auto Create Statistics option is enabled for the database².

Understanding and managing statistics is essential for maintaining the performance of SQL Server databases. They play a key role in how the Query Optimizer handles queries and can significantly impact the efficiency of data retrieval operations.

(1) Statistics – SQL Server | Microsoft Learn. https://learn.microsoft.com/en-us/sql/relational-databases/statistics/statistics?view=sql-server-ver16.
(2) Fundamentals of SQL Server Statistics – SQL Shack. https://www.sqlshack.com/fundamentals-of-sql-server-statistics/.
(3) What are SQL Server Statistics and Where are they Stored?. https://www.sqlnethub.com/blog/what-are-sql-server-statistics-and-where-are-they-stored/.
(4) Managing SQL Server Statistics – Simple Talk – Redgate Software. https://www.red-gate.com/simple-talk/databases/sql-server/performance-sql-server/managing-sql-server-statistics/.
(5) How to Identify Useful SQL Server Table Statistics. https://www.mssqltips.com/sqlservertip/4269/how-to-identify-useful-sql-server-table-statistics/.