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What is a trace flag in SQL Server?

Published in SQL Server Trace Flags 3 mins read

In SQL Server, a trace flag is a configuration setting used to control specific operational characteristics or behaviors of the database engine. According to the reference, trace flags are "used to set specific server characteristics or to alter a particular behavior".

Trace flags provide a way to temporarily change server behavior or enable diagnostic features that are not enabled by default. They can be used for various purposes, such as:

  • Diagnosing Performance Issues: Enabling detailed output or specific logging for troubleshooting.
  • Altering Behavior: Changing how the query optimizer works, how locks are handled, or other internal engine processes.
  • Controlling Features: Enabling or disabling specific features or fixes.

Trace flags are available across different SQL Server platforms, including SQL Server and Azure SQL Managed Instance, as noted in the reference.

How Trace Flags Are Used

Trace flags can be enabled or disabled using the DBCC TRACEON and DBCC TRACEOFF commands, respectively. They can be applied at different scopes:

  • Session Scope: Affects only the current connection or session. Enabled with DBCC TRACEON (TraceFlag, -1).
  • Global Scope: Affects all connections and sessions on the server. Enabled with DBCC TRACEON (TraceFlag, -1). Note: The -1 specifies global scope.
  • Startup Parameters: Can be specified as a startup parameter (-TTraceFlag) for the SQL Server service, enabling the trace flag globally automatically when the instance starts.

It's important to use trace flags with caution, as some can significantly impact server behavior and performance. They are often recommended by Microsoft support to address specific issues.

Common Trace Flag Examples

While the reference doesn't list specific flags, here are a few examples to illustrate their use:

  • Trace Flag 1222: Provides detailed deadlock information in the SQL Server error log.
  • Trace Flag 2371: Alters the threshold for automatic update of statistics based on the number of rows changed (used in older versions or specific scenarios).
  • Trace Flag 4199: Enables optimizer hotfixes released since SQL Server 2005 SP1 that were previously under different trace flags.

Using a trace flag typically involves:

  1. Identifying the specific trace flag number required.
  2. Using DBCC TRACEON to enable it (specifying session or global scope).
  3. Performing the necessary tests or operations.
  4. Using DBCC TRACEOFF to disable it when no longer needed.
Command Purpose Scope Parameter
DBCC TRACEON Enable trace flag Session (none)
DBCC TRACEON Enable trace flag Global (-1)
DBCC TRACEOFF Disable trace flag Session (none)
DBCC TRACEOFF Disable trace flag Global (-1)
DBCC TRACESTATUS Check status Trace Flag Number
Startup Parameter Enable at startup -TTraceFlag

Understanding trace flags is crucial for advanced SQL Server administration and troubleshooting, allowing administrators to fine-tune server behavior or diagnose complex issues.

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