Welcome to our journal article on the topic of update statistics in SQL Server. In this article, we will provide a detailed overview of update statistics in SQL Server, including its importance, how it works, and how to perform it. If you are a database administrator or developer, then this article is for you. Keep reading to learn more.
What are statistics in SQL Server?
Before we dive into update statistics, let’s first understand what statistics are in SQL Server. In SQL Server, statistics are used by the query optimizer to create query execution plans. Statistics contain information about the distribution of values in one or more columns of a table or indexed view. They help the query optimizer choose the most efficient execution plan for a query.
Statistics are created automatically by SQL Server when an index is created. SQL Server also updates the statistics automatically when data in a table changes. However, there may be cases where you need to update statistics manually.
Why is updating statistics important?
Updating statistics is important because it can improve query performance. When the query optimizer creates a query execution plan, it uses statistics to estimate the number of rows that will be returned by a query. If the statistics are out-of-date, the query optimizer may choose a suboptimal execution plan, resulting in slower query performance.
In addition, when you add or delete a large amount of data to a table, the statistics may become stale. Stale statistics can also lead to suboptimal query performance. Therefore, it is important to update statistics after significant changes to the data.
How does SQL Server update statistics?
SQL Server updates statistics using a sampling algorithm. The algorithm scans a portion of the table or indexed view and calculates the distribution of values in the selected columns. The sampling rate can be specified using the UPDATE STATISTICS statement.
SQL Server also provides the option to perform a full scan of the table or indexed view instead of a sample scan. This can be done using the WITH FULLSCAN option of the UPDATE STATISTICS statement. A full scan can be useful when the table or indexed view has been significantly changed and existing statistics are no longer accurate.
When should you update statistics?
As mentioned earlier, SQL Server automatically updates statistics when data in a table changes. However, there may be cases where you need to update statistics manually. Some common scenarios where you may want to update statistics manually include:
| Scenario | Reason to update statistics |
|---|---|
| Large amount of data added or deleted | To avoid stale statistics and improve query performance |
| Table or indexed view has become significantly larger | To improve query performance |
| New indexes have been added to the table or indexed view | To optimize query execution plans |
| The query optimizer is generating suboptimal plans | To improve query performance |
How to update statistics in SQL Server
Using the UPDATE STATISTICS statement
The most common way to update statistics in SQL Server is by using the UPDATE STATISTICS statement. The syntax of the statement is as follows:
UPDATE STATISTICS table_or_indexed_view_name
You can also specify options such as the sampling rate and the option to perform a full scan:
UPDATE STATISTICS table_or_indexed_view_name [WITH [FULLSCAN | SAMPLE number PERCENT]]
For example, to update statistics for a table named “sales”, you would use the following statement:
UPDATE STATISTICS sales
To update statistics for an indexed view named “sales_view” with a sampling rate of 50%, you would use the following statement:
UPDATE STATISTICS sales_view WITH SAMPLE 50 PERCENT
Using SQL Server Management Studio
You can also update statistics using SQL Server Management Studio. Right-click on the table or indexed view you want to update statistics for and select “Properties”. In the “Statistics” tab, click on the “Update” button. You can then choose the sampling rate and whether to perform a full scan.
FAQs
What happens if I do not update statistics?
If you do not update statistics, the query optimizer may choose suboptimal execution plans, resulting in slower query performance. Therefore, it is important to update statistics regularly, especially after significant changes to the data.
How often should I update statistics?
The frequency of updating statistics depends on the nature of the data and the workload. In general, it is recommended to update statistics at regular intervals, such as weekly or monthly. However, if the data changes frequently or the workload is heavy, you may need to update statistics more frequently.
Can I update statistics for multiple tables at once?
Yes, you can update statistics for multiple tables at once using the sp_autostats system stored procedure. The procedure updates statistics for all tables and indexed views in the current database. The syntax of the procedure is as follows:
EXEC sp_autostats
What is the difference between a sample scan and a full scan?
A sample scan scans only a portion of the table or indexed view to calculate statistics. The sampling rate can be specified using the UPDATE STATISTICS statement. A full scan scans the entire table or indexed view to calculate statistics. A full scan can be useful when the table or indexed view has been significantly changed and existing statistics are no longer accurate.
Can I disable automatic statistics updates?
Yes, you can disable automatic statistics updates using the AUTO_UPDATE_STATISTICS database option. However, it is not recommended to disable automatic updates, as stale statistics can lead to suboptimal query performance.
Conclusion
Updating statistics is an important task for database administrators and developers. It can improve query performance and prevent suboptimal execution plans. In this article, we provided an overview of update statistics in SQL Server, including its importance, how it works, and how to perform it. We also provided some common scenarios where you may want to update statistics manually, as well as FAQs to help you better understand the topic.