SQL Server Query Performance Tuning: Understanding Statistics

What Are Statistics in SQL Server?

Statistics are objects that store statistical information about the distribution of values in one or more columns of a table or indexed view.

The query optimizer uses this information to estimate the number of rows that will be returned by a query predicate, which influences the choice of execution plan.

• Contain histogram data about column values.
• Include density information to estimate row counts for multiple columns.
• Automatically created for indexes and can be created manually for columns.

How SQL Server Creates and Updates Statistics

SQL Server can create statistics automatically when AUTO_CREATE_STATISTICS is enabled.

Statistics are updated automatically when data changes exceed a threshold if AUTO_UPDATE_STATISTICS is enabled.

Manual creation and updating of statistics are possible using T-SQL commands.

• AUTO_CREATE_STATISTICS helps ensure statistics exist for columns used in predicates.
• AUTO_UPDATE_STATISTICS keeps statistics current to reflect data changes.
• Manual updates can be forced with UPDATE STATISTICS or sp_updatestats.

Impact of Statistics on Query Performance

Accurate statistics enable the optimizer to estimate row counts correctly, leading to efficient query plans.

Outdated or missing statistics can cause poor cardinality estimates, resulting in suboptimal plans and slower queries.

Large tables with frequent data modifications require more frequent statistics maintenance.

• Poor estimates may cause unnecessary scans or inefficient joins.
• Updating statistics can improve query plan quality and execution speed.
• Monitoring statistics usage helps identify performance bottlenecks.

Best Practices for Statistics Maintenance

Regularly update statistics on large or heavily modified tables.

Use FULLSCAN option for more accurate statistics when necessary.

Consider disabling AUTO_UPDATE_STATISTICS for critical workloads and update statistics manually during maintenance windows.

• Schedule statistics updates during off-peak hours.
• Use sp_updatestats to update all statistics in a database efficiently.
• Monitor query performance before and after statistics updates.

Example: Creating and Updating Statistics

You can create statistics manually on a column using the CREATE STATISTICS command.

Updating statistics manually can be done with UPDATE STATISTICS specifying the table and statistics name.