Thursday, September 3, 2009

Best Practices for Troubleshooting Slow Running Queries

In this article we will share some of the common reasons for slow-running queries and what your approach should be for identifying and fixing them. The best and the quickest way to address any slow running query should be by first preparing a Troubleshooting Checklist or Troubleshooting Guide and than by using it during slow running query situations. This way it can help any developer or DBA to easily and quickly narrow down the issue. (This troubleshooting guide or checklist should be posted on a central location from where all the DBA's / Developers of your team can access and update when required)

Please note: If you experience something new which is not listed in the below list than you may update your TSG (Troubleshooting Guide) appropriately.

Queries or updates that take more than the expected time to execute can be caused by a variety of reasons. Slow-running queries can be caused by performance problems related to your network or the computer where SQL Server is running. Slow-running queries can also be caused by problems with your physical database design and SQL Server configuration.

Some of the most common reasons for slow-running queries and updates are:

1) CPU could be causing the bottleneck
2) Slow network communication (high latency).
3) Insufficient memory in the server computer, or the memory is not properly configured for SQL Server instance.
4) Insufficient useful statistics on indexed columns.
5) Outdated statistics on indexed columns.
6) Insufficient useful indexes.
7) Lack of useful indexed views.
8) Lack of useful data striping.
9) Improper partitioning of data.

Troubleshooting Checklist (covering most common scenarios)

1) CPU could be causing the bottleneck

What to do?

One of the most common complaints I get is that SQL Server CPU started spiking excessively all of a sudden. A CPU bottleneck that happens suddenly and unexpectedly, without any changes or load on the server may be caused due to several reasons but some of the common reasons are:

1) Non-optimal query plan,
2) A poor SQL Server configuration,
3) Improper Application/Database design,
4) Insufficient hardware resources.


2) Could it be slow network communication (high latency).

What to do?

Investigate if the performance problem is related to a component other than queries? For example, is there a problem with network performance, do you see any latency issue, try copying the files across to measure the performance of your network. Also, try to identify if there are any other components that might be causing or contributing to performance degradation?

What tool to use?
The Windows System Monitor can be used to monitor the performance of SQL Server and non-SQL Server related components.

3) Could it be Insufficient memory on the server computer, or the memory is not properly configured for SQL Server instance?

What to do?

If your investigation reveals that the performance issue is related to queries then Identify which query or set of queries are involved?

What tool to use?
You may use SQL Server Profiler to help identify the slow query or queries.

What’s next?
After you have identified the slow-running query or queries, you can further analyze query performance by producing a query SHOWPLAN, which can be a text, XML, or graphical representation of the query execution plan that the query optimizer generates. You can produce query SHOWPLAN using:
Ø Transact-SQL SET options,
Ø SQL Server Management Studio, or
Ø SQL Server Profiler.

The information gathered by these tools would allow you to determine how a query is executed by the SQL Server query optimizer and which indexes are being used. This could help you determine if performance improvements can be made by rewriting the query, changing the indexes on the tables, or perhaps modifying the database design.

4) Could it be due to Insufficient useful statistics on indexed columns.

What to do?

Investigate if the query was optimized with useful statistics or not? In SQL Server, statistics on the distribution of values in a column are created automatically on the indexed columns. They can also be created on non-indexed columns either manually using SQL Server Management Studio or CREATE STATISTICS statement, or automatically, if the AUTO_CREATE_STATISTICS database option is set to TRUE. These statistics can be used by the query processor to determine the optimal strategy for evaluating a query. Maintaining additional statistics on non-indexed columns involved in join operations can improve query performance.

Alternatively, you can also monitor the query using SQL Server Profiler or the graphical execution plan in SQL Server Management Studio to determine if the query has enough statistics.

What tools to use?
Ø You can use Create Statistics, AUTO_CREATE_STATISTICS for creating and updating indexes. (Look at the below section for resource links)
Ø You can also use SQL Server Profiler to determine if query has enough statistics
Ø You can also use Graphical Execution plan from SQL Server Management studio to determine if query has enough statistics or not.

5) Could it be due to outdated statistics on indexed columns.

What to do?

Check if the query statistics are up to date? Are the statistics automatically updated?
SQL Server would automatically create and update query statistics on indexed columns (as long as automatic query statistic updating is not disabled). Additionally, statistics can be updated on non-indexed columns either manually, using SQL Server Management Studio or the UPDATE STATISTICS statement, or automatically, if the AUTO_UPDATE_STATISTICS database option is set to TRUE.

Tip: Up-to-date statistics are not dependent upon date or time data. If no UPDATE operations have taken place, the query statistics are still up-to-date.

What tool to use?
You may use Auto_Create_Statistics property of the database to enable or disable the creation of statistics automatically.


6) Could it be due to Insufficient useful indexes.

What to do?

Investigate if suitable indexes are available? Investigate if adding one or more indexes can improve query performance?

7) Could it be the lack of useful indexed views.

What to do?

Identify if there are there any data or index hot spots? You may also consider using disk striping which can be implemented by using RAID (redundant array of independent disks) level 0 in which the data is distributed across multiple disk drives.

8) Could it be due to lack of useful data striping.

What to do?

Check if the query optimizer has provided with the best opportunity to optimize a complex query?

9) Could it be due to improper partitioning of data.

What to do?

Check if you have a large volume of data and if you do then you need to partition that data to achieve maximum performance from your queries. Data manageability is the main benefit of partitioning, but if your tables and indexes on them are partitioned similarly, partitioning can also improve query performance greatly.

4 comments:

Vijay Kuma Dhiraj said...

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Jason Clark said...
This comment has been removed by the author.
Jason Clark said...

Tremendous Post! But, Updating statistics is not guaranteed to improve performance; if you read the Article below there is a trade-off between updating stats and query performance http://www.sqlmvp.org/are-you-experiencing-slow-running-queries-in-sql-server/