Published on

April 7, 2005

Understanding Trace Flags in SQL Server

Trace flags are a powerful tool in SQL Server that allow you to configure specific server characteristics or switch off certain behaviors. They are often used for diagnosing performance issues or debugging stored procedures and complex systems. In this article, we will explore how to set the status of trace flags, the effects they have, and how to enable or disable them for sessions.

Setting the Status of Trace Flags

Trace flags in SQL Server can have a status of either “on” or “off”. There are two ways to set the status of a trace flag to “on”. The first method is by executing the command DBCC TRACEON, and the second method is by using a startup parameter for SQL Server. Regardless of how a trace flag is set to “on”, it will remain on until it is manually switched off using DBCC TRACEOFF.

To activate a trace flag using DBCC TRACEON, you can use the following syntax:

DBCC TRACEON(trace#)

Executing this command will mark the specified trace flag as “on”. It’s important to note that trace flags are set globally for the entire server instance, meaning that all sessions that are enabled for using trace flags will be affected by the activated trace flags.

Trace flags can also be set automatically by using the startup parameter -T#, where # represents the number of the trace flag to be set on. This is typically used for flags that are not specific to a user session or flags that the database administrator always wants to have on.

Disabling Trace Flags

To set the status of a trace flag to “off”, you can use the command DBCC TRACEOFF. The syntax for this command is as follows:

DBCC TRACEOFF(trace#)

Executing DBCC TRACEOFF with the trace flag number as an argument will deactivate the corresponding trace flag immediately. It’s important to note that trace flags are always switched off globally, meaning that all sessions, including the current session, will stop being affected by the deactivated trace flags.

Enabling and Disabling Sessions for Trace Flags

A user session in SQL Server can be either enabled or disabled for using trace flags. If a session is enabled, it will be affected by all active trace flags, regardless of how they were set or by which user. If a session is disabled, it will not be affected by any trace flags.

The default setting for new sessions is that they are disabled. However, this default can be changed by certain actions. To check the status of trace flags for a session, you can use the command DBCC TRACESTATUS:

DBCC TRACESTATUS(trace#)

If executed in a session that is enabled for using trace flags, this command will return a result set showing the status of the specified trace flags. If executed in a session that is disabled from using trace flags, it will not return any result set but instead display a message indicating that trace options are not enabled for that connection.

To enable a session for using trace flags, you can use the command DBCC TRACEON with no parameters:

DBCC TRACEON

Executing this command will enable the current session to start using trace flags. It’s important to note that this command does not activate any trace flags, but rather enables the session to use any existing and future active trace flags.

To disable a session from using trace flags, you can use the command DBCC TRACEOFF with no parameters:

DBCC TRACEOFF

Executing this command will disable the current session from using trace flags. Other sessions will not be affected, and the status of any active trace flags will remain unchanged.

Conclusion

Trace flags in SQL Server provide a way to configure server characteristics and control certain behaviors. By understanding how to set the status of trace flags, enable or disable sessions for using them, you can effectively diagnose performance issues and debug complex systems. Remember that trace flags are set globally for the entire server instance, and enabling or disabling a session will affect all active trace flags for that session.

References:

Microsoft SQL Server 2000 Books Online (Updated – 2004)

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