Introduction
One of the key features of SQL Server is the query optimizer, which uses statistics to determine the most efficient execution plan for a query. Statistics are used to estimate the number of rows that will be returned by a query and to determine the most efficient access path to the data. In this article, we will discuss how to update statistics in SQL Server to ensure that the query optimizer has the most up-to-date information to make accurate estimates.
What are Statistics in SQL Server?
Update Statistics is a command or process in SQL Server that updates the statistical information about the distribution of data in one or more tables or indexes in a database. They consist of summary data that allows the query optimizer to make informed decisions about how to retrieve data efficiently. These statistics help SQL Server determine the most optimal query execution plans, which ultimately impacts query performance.
By default, SQL Server automatically updates statistics when a certain threshold of data changes has been reached. This threshold is known as the “auto-update” threshold. However, there are situations where you may want to update statistics manually to ensure that the query optimizer has the most accurate information to work with.
Why are Statistics Important?
Accurate statistics are critical for SQL Server to generate the most efficient execution plan for a query. If the statistics are outdated or inaccurate, the query optimizer may choose a suboptimal execution plan, resulting in slower query performance. Additionally, inaccurate statistics can lead to suboptimal index usage, which can cause excessive disk I/O and memory usage.
When to Update Statistics in SQL Server?
While SQL Server automatically updates statistics for you, there are certain scenarios where you may want to update statistics manually. These scenarios include:
- Large Data Changes: If a large percentage of data has been added, modified, or deleted from a table, you may want to update statistics manually to ensure that the query optimizer has the most accurate information.
- Stale Statistics: If the statistics for a table or indexed view are stale (out of date), you may want to update them manually to ensure that the query optimizer has the most accurate information. This can happen when data is added, modified, or deleted from a table or indexed view after the statistics were last updated.
- Performance Issues: If you are experiencing performance issues with a particular query, updating statistics may help to improve performance by providing the query optimizer with more accurate information.
How to Update Statistics in SQL Server SQL Server?
- Automatic Updates - SQL Server automatically updates statistics when a threshold of data modifications occurs, or when the query optimizer determines that the statistics are out of date. This is the default behavior for SQL Server, and it works well for most databases.
- Manual Updates - We can manually update statistics using the UPDATE STATISTICS statement.
Update the statistics for a single table or indexed view using the following command:
UPDATE STATISTICS [table_name]
Where [table_name] is the name of the table or indexed view for which statistics to be updated.
Update the statistics for all tables in a database using the following command:
EXEC sp_updatestats
When statistics getting updated, SQL Server automatically updates the statistics for all columns that have a density value that is out of date. This means that the update may not necessarily update all statistics for all columns in the table or indexed view.
It's important to note that updating statistics can be a resource-intensive operation, particularly for large tables. Therefore, it's important to carefully consider the frequency and timing of statistics updates to ensure they don't negatively impact system performance.
How to view the statistics property?
By using the below T-SQL command:
DBCC SHOW_STATISTICS
For more information, see DBCC SHOW_STATISTICS
We can use ‘sys.stats’ system catalog view, which provides information about statistics for all user-defined and internal tables and indexes in the current database by using following command:
SELECT
OBJECT_NAME(s.object_id) AS TableName,
i.name AS IndexName,
s.name AS StatisticName,
s.auto_created AS IsAutoCreated,
s.user_created AS IsUserCreated,
STATS_DATE(s.object_id, s.stats_id) AS LastUpdated
FROM sys.stats AS s
INNER JOIN sys.indexes AS i
ON s.object_id = i.object_id
AND s.stats_id = i.index_id
WHERE OBJECTPROPERTY(s.object_id, 'IsUserTable') = 1
ORDER BY TableName, IndexName
This statement retrieves information about statistics for all user-defined tables and indexes in the database, including the table and index names, statistic name, whether the statistic was automatically or manually created, and the date and time the statistics were last updated.
Examine the “LastUpdated” column to determine whether the statistics are up-to-date. If the value in this column is relatively recent, the statistics are up-to-date. However, if the value is old or NULL, the statistics may be out-of-date and need to be updated.
Conclusion
Statistics are an important component of SQL Server's query optimization process. By providing the query optimizer with accurate information about the distribution of data in a table or indexed view, statistics can help to improve query performance. While SQL Server automatically updates statistics for you, there are certain scenarios where you may want to update them manually to ensure that the query optimizer has the most accurate information. By understanding when and how to update statistics in SQL Server, you can help to ensure that your queries are running as efficiently as possible.