Hello, and welcome to our comprehensive guide on updating SQL Server statistics. In this article, we will explore everything you need to know about SQL Server statistics, why they are important, and how to keep them up to date. Whether you are a database administrator, a developer, or simply someone looking to improve the performance of your SQL Server database, this guide is for you.
Table of Contents
- Introduction
- What are SQL Server Statistics?
- Why Are SQL Server Statistics Important?
- How to Update SQL Server Statistics
- When to Update SQL Server Statistics
- Best Practices for Updating SQL Server Statistics
- Common Questions About SQL Server Statistics
- Conclusion
Introduction
If you are working with a SQL Server database, you have probably heard about SQL Server statistics. But what are they, exactly?
In short, SQL Server statistics are metadata about the distribution of values in one or more columns of a table or an indexed view. They are used by the SQL Server Query Optimizer to choose the most efficient execution plan for a given query.
In this guide, we will explore everything you need to know about SQL Server statistics, why they are important, and how to keep them up to date.
What are SQL Server Statistics?
SQL Server statistics are metadata about the distribution of values in one or more columns of a table or an indexed view. They are created automatically by SQL Server when an index is created or when the AUTO_CREATE_STATISTICS database option is set to ON.
SQL Server statistics are stored in a special system table in the database called sys.stats. Each statistics object is associated with one or more columns and a histogram that describes the distribution of values in those columns.
Types of SQL Server Statistics
There are two types of SQL Server statistics:
- Index statistics: These statistics are created automatically when an index is created. They are associated with the columns in the index key and include information about the distribution of values in those columns.
- Column statistics: These statistics are automatically created by SQL Server when a query is executed and the optimizer determines that the statistics do not exist or are out of date. They are associated with one or more columns and include information about the distribution of values in those columns.
How SQL Server Statistics are Used
The SQL Server Query Optimizer uses statistics to estimate the number of rows that will be returned by a query, as well as the distribution of values in those rows. This information is used to choose the most efficient execution plan for the query.
For example, suppose you have a table with a million rows and a column called “Status” that has two possible values: “Active” and “Inactive”. If you execute a query that filters the “Status” column by “Active”, the optimizer will use the statistics for the “Status” column to estimate the number of rows that match the filter condition. If the statistics show that 70% of the rows have a “Status” of “Active”, the optimizer will estimate that the query will return around 700,000 rows.
Why Are SQL Server Statistics Important?
SQL Server statistics are critical to the performance of your database. Without up-to-date statistics, the SQL Server Query Optimizer may not choose the most efficient execution plan for your queries, resulting in slower performance and longer query times.
When SQL Server creates statistics objects, it uses a sample of the data in the column to create the histogram. This means that if the distribution of values in the column changes significantly, the statistics may become out of date. If the statistics are out of date, the Query Optimizer may not choose the most efficient execution plan for your queries, resulting in suboptimal performance.
How to Update SQL Server Statistics
There are several ways to update SQL Server statistics:
Automatic Updating
SQL Server can automatically create and update statistics for you, using the AUTO_CREATE_STATISTICS and AUTO_UPDATE_STATISTICS database options. If these options are set to ON, SQL Server will automatically create and update statistics as needed.
However, automatic updating may not always be the best option. For example, if you have a very large table with a lot of write activity, automatic updating may cause performance issues. In this case, you may want to consider manual updating.
Manual Updating
You can manually update SQL Server statistics using the UPDATE STATISTICS statement or the sp_updatestats stored procedure. The syntax for the UPDATE STATISTICS statement is as follows:
| UPDATE STATISTICS | <Table or indexed view name> | [ | <Statistics name> | ] | [WITH [<update_option> [, … ]]] |
|---|
The UPDATE STATISTICS statement updates statistics for the specified table or indexed view. If you do not specify a statistics name, SQL Server updates all statistics objects for the table or indexed view.
The sp_updatestats stored procedure updates statistics for all tables and indexed views in the current database. You can also specify a table or indexed view name as a parameter to the stored procedure to update statistics for a specific object.
When to Update SQL Server Statistics
There are several situations in which you may want to update SQL Server statistics:
- After bulk data modifications: If you perform a bulk insert, update, or delete operation that changes more than 20% of the rows in a table, you may want to update the statistics for the affected columns.
- After index maintenance: If you rebuild or reorganize an index, SQL Server automatically updates the statistics for the indexed columns. However, if you disable automatic statistics updating or create an index with the DROP_EXISTING option, you will need to manually update the statistics.
- After significant data changes: If the data in a column changes significantly, you may want to update the statistics to ensure that the Query Optimizer is using up-to-date information.
Best Practices for Updating SQL Server Statistics
Here are some best practices for updating SQL Server statistics:
- Use automatic updating where possible: If your database is small or has low write activity, you may want to consider using automatic statistics updating. This will ensure that your statistics are always up to date without requiring manual intervention.
- Update statistics regularly: Even if you use automatic updating, it is a good idea to manually update statistics on a regular basis. This will ensure that your statistics are up to date and may improve the performance of your queries.
- Monitor query performance: Keep an eye on the performance of your most frequently executed queries. If you notice a slowdown, check the execution plan to see if the Query Optimizer is choosing a suboptimal plan. If so, consider updating the statistics for the affected columns.
- Consider using filtered statistics: If you have a large table with a column that has a skewed distribution of values, consider creating filtered statistics for the most commonly used values. This can improve query performance by giving the Query Optimizer more accurate information about the distribution of values in the column.
Common Questions About SQL Server Statistics
What is the difference between index statistics and column statistics?
Index statistics are created automatically when an index is created. They are associated with the columns in the index key and include information about the distribution of values in those columns. Column statistics are automatically created by SQL Server when a query is executed and the optimizer determines that the statistics do not exist or are out of date. They are associated with one or more columns and include information about the distribution of values in those columns.
When should I update SQL Server statistics?
You should update SQL Server statistics after bulk data modifications, after index maintenance, and after significant data changes. It is also a good idea to update statistics regularly to ensure that they are up to date.
How do I update SQL Server statistics?
You can update SQL Server statistics using the UPDATE STATISTICS statement or the sp_updatestats stored procedure.
What are best practices for updating SQL Server statistics?
Best practices for updating SQL Server statistics include using automatic updating where possible, updating statistics regularly, monitoring query performance, and considering using filtered statistics.
Conclusion
In conclusion, SQL Server statistics are a critical component of your database’s performance. By ensuring that your statistics are up to date and following best practices for updating them, you can improve the performance of your queries and ensure that your database runs efficiently.
We hope that this comprehensive guide has answered all of your questions about SQL Server statistics and provided you with the information you need to keep your database running smoothly. If you have any further questions, please feel free to contact us.