Wait Type WAIT_ON_SYNC_STATISTICS_REFRESH

Wait statistics, in the context of SQL Server, refer to the amount of time that a query spends waiting to access data in the database. When a client application requests data from the database, the request is placed in a queue and the client application must wait for its turn to access the data. The time that the query spends waiting is called a "wait" and is tracked by SQL Server. This information can be used to identify potential performance bottlenecks and optimize the performance of the database. Wait statistics are commonly used by database administrators to diagnose and troubleshoot performance issues in SQL Server.

In SQL Server, wait types help us identify areas where tasks may be delayed. One specific wait type that often shows up during query optimization is WAIT_ON_SYNC_STATISTICS_REFRESH. This wait type appears when SQL Server is waiting for statistics to be updated, or “refreshed,” to provide the most accurate data for the query execution plan. In this post, we’ll break down what WAIT_ON_SYNC_STATISTICS_REFRESH means, why it happens, and how it might affect SQL Server performance.

What Is the WAIT_ON_SYNC_STATISTICS_REFRESH Wait Type?

The WAIT_ON_SYNC_STATISTICS_REFRESH wait type occurs when SQL Server is waiting for an update to the statistics on a table or index. Statistics in SQL Server are essential for query optimization because they contain information about the distribution of data in tables, such as the number of rows, ranges, and data uniqueness. SQL Server relies on these statistics to choose the most efficient way to execute a query.

When statistics are outdated, SQL Server may choose a less efficient execution plan. To avoid this, SQL Server may trigger a statistics update before running the query, and it waits for this update to finish. During this time, it records a WAIT_ON_SYNC_STATISTICS_REFRESH wait, indicating that the system is waiting for the latest statistics to be available.

When Does WAIT_ON_SYNC_STATISTICS_REFRESH Appear?

The WAIT_ON_SYNC_STATISTICS_REFRESH wait type typically appears when SQL Server detects that statistics are out of date and automatically updates them before executing a query. Common scenarios where this wait type may show up include:

• First Query After Data Changes – When a query is run after significant data changes (such as many inserts or updates), SQL Server may need to refresh statistics to ensure accurate query optimization.
• Large Tables with Auto-Update Statistics Enabled – For large tables, automatic statistics updates can take more time, causing a WAIT_ON_SYNC_STATISTICS_REFRESH wait while SQL Server completes the update.
• Complex Queries or Reports – Complex queries that involve many tables and indexes may need fresh statistics across multiple objects to optimize effectively, leading to this wait type.

These waits are normal and expected in environments with frequent data changes, as SQL Server keeps statistics up to date to improve query performance.

Why WAIT_ON_SYNC_STATISTICS_REFRESH Waits Matter

While WAIT_ON_SYNC_STATISTICS_REFRESH waits are expected in SQL Server, prolonged or frequent waits can impact query performance, especially for time-sensitive queries. High WAIT_ON_SYNC_STATISTICS_REFRESH wait times may indicate:

• High Volume of Data Changes – In environments with frequent or large data modifications, statistics may become outdated more often, leading to frequent updates.
• Large Table Sizes – Updating statistics on large tables takes more time, increasing the wait time for queries that require these updates.
• Auto-Update Statistics Settings – Automatic statistics updates can be useful but may cause delays if triggered frequently, especially on larger tables or indexes.

How to Address WAIT_ON_SYNC_STATISTICS_REFRESH Waits

If WAIT_ON_SYNC_STATISTICS_REFRESH waits are affecting query performance, consider these strategies to reduce their impact:

• Enable Asynchronous Statistics Updates – Setting statistics updates to asynchronous allows queries to run without waiting for statistics to refresh, which can reduce delays during query execution.
• Manually Update Statistics – For tables with high data modification, manually updating statistics during off-peak hours can reduce the need for auto-updates during critical query times.
• Optimize Auto-Update Statistics Thresholds – For large tables, adjusting the auto-update statistics threshold can reduce the frequency of updates, minimizing potential delays.
• Monitor Large Tables and Indexes – Keep an eye on tables with heavy data changes and adjust statistics update schedules as needed to ensure efficient query performance.

Conclusion

The WAIT_ON_SYNC_STATISTICS_REFRESH wait type in SQL Server occurs when the system is waiting for statistics to be refreshed, ensuring accurate query optimization. While these waits are a normal part of SQL Server’s operation, prolonged or frequent waits can indicate high data modification rates or large table sizes that require regular statistics updates. By enabling asynchronous statistics updates, adjusting auto-update thresholds, and scheduling manual statistics updates during low-activity periods, you can help reduce WAIT_ON_SYNC_STATISTICS_REFRESH waits and improve overall query performance.

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Applies to

• SQL Server 2022

See Also

• More About WAIT_ON_SYNC_STATISTICS_REFRESH