Understanding SQL Server Auto Stats Behavior
By Tom Nonmacher
In the world of database management systems, understanding how systems optimize performance is key to maintaining efficient operations. One of the key features of SQL Server 2016, SQL Server 2017, MySQL 5.7, DB2 11.1, and Azure SQL that assists in performance optimization is Auto Stats. This feature, available in all the aforementioned technologies, is designed to automatically update the statistics of the database, ensuring that query performance remains at an optimum level.
The SQL Server engine uses statistics to create query plans that improve performance. When the Auto Stats feature is enabled, SQL Server automatically updates these statistics based on the number of data modifications in the database. This includes data insertions, updates, deletions, and merges. When a certain threshold of changes is reached, SQL Server triggers an automatic update of the statistics.
To demonstrate this, let's consider a simple example in T-SQL, where we check the current Auto Stats status for a particular database.
USE YourDatabaseName;
GO
SELECT name, is_auto_create_stats_on, is_auto_update_stats_on
FROM sys.databases WHERE name = 'YourDatabaseName';
The returned value of '1' for 'is_auto_create_stats_on' and 'is_auto_update_stats_on' indicates that Auto Stats is enabled for that particular database.
In MySQL, the equivalent to Auto Stats is the 'innodb_stats_auto_recalc' option. This functionality allows InnoDB to automatically recalculate persistent statistics once the data in the table has changed significantly. You can check the status of this feature using the following SQL statement:
SHOW VARIABLES LIKE 'innodb_stats_auto_recalc';
The returned value of 'ON' indicates that the feature is enabled.
In DB2 11.1, the Automatic Statistics feature is enabled by default. This feature allows DB2 to collect statistics on tables and indexes automatically. You can check the status of this feature using the following SQL statement:
SELECT AUTO_STATS_MODE FROM TABLE (SYSPROC.ADMIN_GET_DB_CFG()) AS T;
The returned value of 'ON' or 'OFF' indicates the current status of the Automatic Statistics feature.
In Azure SQL, the Auto Stats feature is always enabled and cannot be disabled. This is part of Azure SQL's design to manage performance in the background, without requiring manual intervention.
In conclusion, understanding how Auto Stats work across different database technologies can help you maintain efficient database operations. By allowing the database engine to automatically update the statistics, you can ensure that your queries are always optimized for the current data distribution within your database.