Implement statistics maintenance tasks

The implementation of statistics maintenance tasks is crucial for effectively administering Microsoft Azure SQL solutions. Statistics play a vital role in query optimization and performance tuning, ensuring the database engine can make informed decisions on query execution plans. In this article, we will explore the process of implementing statistics maintenance tasks in Azure SQL.

1. Understanding Statistics

Before diving into the implementation, let’s briefly understand what statistics are. Statistics provide the SQL Server query optimizer with information about the distribution of data in a column or columns used in queries. This information helps the optimizer estimate the number of rows returned by a query and select the most efficient execution plan.

2. Automating Statistics Maintenance

Azure SQL offers two methods to automate statistics maintenance: automatic statistics update and automatic statistics asynchronous update.

• Automatic Statistics Update:

By default, Azure SQL automatically updates statistics when a threshold for data changes is reached. However, you can adjust this threshold by modifying the AUTO_UPDATE_STATISTICS_ASYNC option using the following code:

ALTER DATABASE [DatabaseName] SET AUTO_UPDATE_STATISTICS_ASYNC ON;

This option allows automatic statistics updates to occur asynchronously, reducing query latency. Additionally, consider enabling the AUTO_CREATE_STATISTICS option for certain columns to guarantee the creation of statistics objects when needed.

• Automatic Statistics Asynchronous Update:

To enable asynchronous statistics updates, run the following SQL command:

ALTER DATABASE [DatabaseName] SET AUTO_UPDATE_STATISTICS_ASYNC OFF;

Asynchronous statistics updates ensure that queries don’t wait for statistics updates to complete, enhancing overall database performance in certain scenarios.

3. Manual Statistics Maintenance

While automatic statistics maintenance helps keep your statistics up to date, there might be cases where you need to perform manual maintenance. This is especially true when specific columns or tables require special attention.

• Manual Statistics Update:

To manually update statistics, use the following SQL command:

UPDATE STATISTICS [SchemaName].[TableName] [IndexName | StatisticsName];

Replace [SchemaName].[TableName] with the appropriate schema and table name. You can also specify an index or statistics name if needed.

• Manual Statistics Full Scan:

In some cases, a full scan of the table is required to update statistics accurately. Use the WITH FULLSCAN option as shown below:

UPDATE STATISTICS [SchemaName].[TableName] WITH FULLSCAN;

This command ensures a full scan of the data to generate up-to-date statistics.

• Manual Statistics Sampled Update:

If you prefer a sampled update rather than a full scan, you can specify the SAMPLE percentage using the following code:

UPDATE STATISTICS [SchemaName].[TableName] WITH SAMPLE [SamplePercentage];

Replace [SamplePercentage] with the desired percentage of rows to sample.

4. Maintenance Plan and SQL Agent Jobs

To automate the manual statistics maintenance process, you can create a maintenance plan using SQL Server Management Studio (SSMS). Here’s a basic outline:

• Open SSMS, right-click the database, and select “Tasks” > “Manage Database Maintenance Plans.”
• Follow the wizard to create a new plan, specifying “Update Statistics” as the maintenance task.
• Configure the task according to your requirements, such as the scope (all tables or specific tables), update options (full scan or sampled), and scheduling frequency.

Once the maintenance plan is created, you can use SQL Agent Jobs to schedule it to run at specific intervals automatically.

By following these steps and understanding the concepts discussed, you can effectively implement statistics maintenance tasks in Azure SQL to optimize query performance and ensure accurate data distribution estimates for the query optimizer.