Default Trace–Performance Issues

There are multiple events that a default trace in SQL Server 2005 and above tracks which can be significantly useful for finding out areas of improvement. The events that I will be concentrating on are:

1. Missing Column Statistics – This event class indicates that column statistics that could have been useful for the optimizer are not available due to which an incorrect cardinality estimation could occur. This can cause the optimizer to choose a less efficient query plan than expected. You will not see this event produced unless the option to auto-create statistics is turned off.

2. Missing Join Predicate – This event class indicates that a query is being executed that has no join predicate. (A join predicate is the ON search condition for a joined table in a FROM clause.) This could result in a long-running query. This event is produced only if both sides of the join return more than one row.

3. Sort Warnings – This event class indicates that sort operations do not fit into memory. This does not include sort operations involving the creation of indexes, only sort operations within a query (such as an ORDER BY clause used in a SELECT statement). The EventSubClass field in this event shows whether this was a single pass or a multiple pass. A single pass (EventSubClass = 1) is when the sort table was written to disk, only a single additional pass over the data was required to obtain sorted output. A multiple pass (EventSubClass = 2) is when the sort table was written to disk, multiple passes over the data were required to obtain sorted output. A multiple pass is an enemy of query performance.

4. Hash Warnings – This event class can be used to monitor when a hash recursion or cessation of hashing (hash bailout) has occurred during a hashing operation.  Hash recursion (EventSubClass = 0) occurs when the build input does not fit into available memory, resulting in the split of input into multiple partitions that are processed separately. Hash bailout (EventSubClass = 1) occurs when a hashing operation reaches its maximum recursion level and shifts to an alternate plan to process the remaining partitioned data. Hash bailout usually occurs because of skewed data. Another enemy of performance!

5. Server Memory Change – This event class occurs when Microsoft SQL Server memory usage has increased or decreased. You can even determine what is the current memory usage after the increase or decrease.

6. Log File Auto Grow – This event class indicates that the log file grew automatically. This event is not triggered if the log file is grown explicitly through ALTER DATABASE. Frequent log file growths are not food for performance.

7. Data File Auto Grow – This event class indicates that the data file grew automatically. This event is not triggered if the data file is grown explicitly by using the ALTER DATABASE statement.

Since this information is already available in the default trace, I decided to use my Default Trace Statistics Power View Excel sheet to track this information graphically. And this is what I got (see screenshot 1)!


So what is the above Excel sheet displaying?

1. The information available in the first column chart will show the Data and Log file grow events per database.

2. The first matrix in the middle of the Excel sheet shows the number of Sort Warnings and Hash Warnings with drill-down capabilities for each database to see the EventSubClass fields.

3. The second matrix shows the Missing Column Statistics and the Missing Join Predicate events for each database. The drill-down capability gives the name of the column statistics that was missing.

4. The line graph shows the change in memory for the SQL Server database engine.

Happy monitoring!

Previous posts in this series:

Schema Changes History Report


SQL Bangalore UG Meet

I had the opportunity to speak at the SQL Bangalore UG Meet on 22nd December 2012. A big thank you to Vinod Kumar (B | T) for organizing this meet!!

The topic that I presented on was the SQL Server System Health Session which has been there since SQL Server 2008 and has been further enhanced in SQL Server 2008.

The MSDN documentation on the System Health Session is available here:

All the System Health Session articles that I have written are available in this summary post: (The RDL files for the reports demoed can be downloaded from here)

System Health Session Custom Reports for management studio can be downloaded from here: (Note that the custom SSMS reports only work for SQL Server 2008 and SQL Server 2008 R2 instances)

Presentation delivered at the meet is available above!

As usual it was good fun presenting at the session! And what I really like about such sessions is that I get to meet the #SQLFamily in person!!


Creating custom XEvent Templates

SQL Server 2012 added the Extended Events UI which made working with Extended Events session much easier. You no longer need to chalk up lengthy T-SQL scripts to spawn off your Extended Events monitoring session. The UI also provides you an option to view the collected data and perform groupings and aggregations for easier analysis.

The templates that you see while configuring an Extended Event session are located at “C:\Program Files (x86)\Microsoft SQL Server\110\Tools\Templates\sql\xevent“. When you export an Extended Events session, you will be given an option to export the Session as a Template to the following folder: “C:\Users\<user name>\Documents\SQL Server Management Studio\Templates\XEventTemplates“. The template is always exported as an XML file. Now when you create a new Extended Event Session, you will see the template under the user templates section in the templates view as shown in the screen shot below:

You can now choose to create as many templates as you want for your monitoring purposes. However, the template files have to be deposited in the above mentioned location so that Management Studio can show them to you while creating an Extended Events session.

The above blog post is a manifestation of a seemingly simple question asked by Naman Vadhera yesterday while he was exploring the Extended Events UI option.