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How Do I Add a Trace Flag?

Published in SQL Server Administration 4 mins read

Adding a trace flag in SQL Server can be done in several ways, depending on whether you want the flag to apply globally, to a specific session, or just to a single query.

Trace flags are used to temporarily set specific server characteristics or to switch off a particular behavior. They are often used for diagnosing performance issues or debugging stored procedures.

Here are the primary methods for adding a trace flag:

Methods for Setting Trace Flags

You can enable a trace flag using the following approaches, each with a different scope and persistence:

Using the DBCC TRACEON Command

This is the most common way to enable a trace flag interactively.

  • Scope: Can be applied globally (for all sessions) or only to the current session.
  • Persistence: The trace flag remains active until explicitly turned off with DBCC TRACEOFF, the session ends (if not global), or the SQL Server service is restarted.

Syntax:

DBCC TRACEON (trace_flag [, -1])
  • trace_flag: The number of the trace flag you want to enable (e.g., 1204, 2528).
  • -1: Optional parameter. If -1 is specified, the trace flag is enabled globally for all sessions. If omitted, the flag is enabled only for the current session.

Examples:

  • Enable trace flag 1204 for the current session:
    DBCC TRACEON (1204);
GO
  • Enable trace flag 2528 globally for all sessions:
    DBCC TRACEON (2528, -1);
GO

To remove a trace flag using this method, you use DBCC TRACEOFF:

DBCC TRACEOFF (trace_flag [, -1])
GO

Using the -T Startup Option

This method ensures that a trace flag is active immediately when the SQL Server service starts.

  • Scope: Applied globally to all sessions from startup.
  • Persistence: The trace flag is active as long as the SQL Server service is running. It will be active automatically upon subsequent restarts.

How to Use:

  1. Open SQL Server Configuration Manager.
  2. Navigate to SQL Server Services.
  3. Right-click on the SQL Server instance you want to configure (e.g., SQL Server (MSSQLSERVER) or SQL Server (YourInstanceName)) and select Properties.
  4. Go to the Startup Parameters tab.
  5. In the "Specify a startup parameter" box, type -T followed by the trace flag number (e.g., -T1222 or -T4199).
  6. Click Add.
  7. Click Apply and then OK.
  8. Restart the SQL Server service for the change to take effect.

Note: Adding startup parameters requires careful consideration and should only be done by administrators.

Using the QUERYTRACEON Query Hint

This method allows you to apply a specific trace flag only to a single query.

  • Scope: Applies only to the specific query where the hint is used.
  • Persistence: Active only for the duration of that single query's execution.

Syntax:

You include this hint within the OPTION clause of your SELECT, INSERT, UPDATE, or DELETE statement.

SELECT column1, column2
FROM your_table
WHERE condition
OPTION (QUERYTRACEON trace_flag);

Example:

  • Apply trace flag 2312 to a specific SELECT query:
    SELECT SalesOrderID, OrderDate, DueDate
FROM Sales.SalesOrderHeader
WHERE SalesOrderID = 43659
OPTION (QUERYTRACEON 2312);
GO

This method is particularly useful for testing the effect of a specific trace flag on a problematic query without affecting the entire server or other sessions.

By understanding these three methods, you can choose the appropriate way to apply trace flags based on your specific needs and the required scope and duration.

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