Update Statistics in SQL Server

Hello Dev! In this article, we will discuss the importance of updating statistics in SQL Server and how to do it effectively. As you know, statistics play a crucial role in optimizing query performance, and updating them is necessary to maintain the accuracy of query execution plans.

What are Statistics in SQL Server?

Before we dive into updating statistics, let’s understand what they are. Statistics are objects that contain information about the distribution of values in one or more columns of a table or an index. SQL Server uses these statistics to estimate the cardinality of a query result set, which helps optimize query plans.

Statistics can be created on one or more columns of a table or index, and SQL Server automatically creates them when an index is created. However, statistics can also be created manually using the CREATE STATISTICS statement.

Why are Statistics Important?

Statistics are crucial for optimizing query performance, as they help the query optimizer choose the most efficient query execution plan. Without accurate statistics, the query optimizer may choose a suboptimal plan, leading to slower query performance.

Furthermore, as data changes over time, the statistics may become outdated, leading to further degradation of query performance. Hence, it is essential to regularly update statistics to ensure accurate cardinality estimates and optimal query plans.

How to Update Statistics

Now that we know the importance of updating statistics, let’s see how to do it effectively. SQL Server provides several methods to update statistics, which we will discuss in this section.

Using the UPDATE STATISTICS Statement

The easiest way to update statistics is by using the UPDATE STATISTICS statement. This statement updates the statistics of a table or an index with the latest information from the underlying data.

Syntax
Update statistics [schema_name.]table_name [index_name]	[WITH [FULLSCAN | SAMPLE number_rows | RESAMPLE]]

The schema_name and index_name parameters are optional and specify the table schema and index name, respectively. The WITH clause is also optional and allows you to specify the scan type and sample size for the update operation.

Using the sp_updatestats System Stored Procedure

Another way to update statistics is by using the sp_updatestats system stored procedure. This procedure updates all statistics in the current database that have been marked for update by SQL Server. By default, SQL Server marks statistics for update when more than 20% of the underlying data has changed.

Syntax
EXEC sp_updatestats

Using the Maintenance Plan Wizard

If you prefer a graphical user interface, you can use the Maintenance Plan Wizard to create a maintenance plan that includes updating statistics. This wizard allows you to schedule periodic updates of statistics and other maintenance tasks, such as database backups and index rebuilds.

To use the Maintenance Plan Wizard, follow these steps:

1. Open SQL Server Management Studio and connect to the SQL Server instance.
2. Expand the Management node and right-click on Maintenance Plans.
3. Select Maintenance Plan Wizard from the context menu.
4. Follow the wizard steps to create a new maintenance plan and select the Update Statistics task.
5. Specify the frequency and other options for the maintenance plan.
6. Save the maintenance plan and execute it.

When to Update Statistics

Now that we know how to update statistics, let’s discuss when to do it. Updating statistics too frequently or infrequently can both have negative effects on query performance.

As a general rule, you should update statistics when:

• The table has undergone significant changes in data volume or distribution
• The query optimizer is choosing suboptimal execution plans
• The query performance has significantly degraded

It is also recommended to update statistics during off-peak hours to minimize the impact on production workloads.

FAQs

Q. How often should I update statistics?

A. The frequency of updating statistics depends on the rate of data change in the table and the query workload. A good practice is to update statistics when the data changes by 20% or more and monitor query performance for any degradation.

Q. What is the difference between a full scan and a sampled scan?

A. A full scan reads all the rows in the table or index and provides accurate statistics, but it can be resource-intensive for large tables. A sampled scan reads a random sample of rows and provides approximate statistics, but it is faster and less resource-intensive.

Q. Can I update statistics on a table with no indexes?

A. Yes, you can update statistics on a table with no indexes using the UPDATE STATISTICS statement or the sp_updatestats procedure. However, without indexes, the data is likely to be scanned frequently, leading to slower query performance.

Q. Do I need to update statistics on a partitioned table?

A. Yes, you should update statistics on each partition of a partitioned table separately. SQL Server provides a special syntax to update statistics on partitioned tables.

Q. How can I monitor the statistics update process?

A. You can monitor the statistics update process by using the sys.dm_exec_requests and sys.dm_exec_sessions dynamic management views. These views provide information about the status, progress, and resource usage of the update operation.

Conclusion

Updating statistics is a critical maintenance task for optimizing query performance in SQL Server. In this article, we discussed the importance of statistics, how to update them using various methods, and when to update them. By following these best practices, you can ensure accurate cardinality estimates, optimal query plans, and faster query performance.