SQL Server Magic Buttons! What are Trace Flags and why should I care? Steinar Andersen, SQL Service Nordic AB Thanks to Thomas Kejser for peer-reviewing

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About me  SQL Server nerd since 1995  SQL Server 4.21, 6.0, 6.5, 7.0, 2000, 2005, 2008, 2012, 2014, 2016, Azure  Former Member of the Board SQLUG (sqlug.se)  Blog:   Speaker at:  TechDays  Microsoft Sommarkollo  Cloud Seminarium  SQL Saturday

What are Trace Flags?  Trace Flags are settings that in some way or another alters the behavior of various SQL Server functions.  Is inherited from Sybase SQL Server code  Some trace flags are still the same such as 1204, 3604, 3605, 3607, 3608  REMEMBER: Be extremely careful with trace flags, test in your test environment first. And consult professionals first if you are the slightest uncertain about the effects of your changes.

Why should I care?  It could save you some day, like it saved me…  msdb110_upgrade-sql-encountered-error-547/ msdb110_upgrade-sql-encountered-error-547/

How do I use Trace Flags?  How do I turn Trace Flags on and off? *1  – You can use the DBCC TRACEON and DBCC TRACEOFF commands  – You can use the -T option in the startup configuration for the SQL Server Service  -You can also use the hint QUERYTRACEON in your queries: ::={ QUERYTRACEON trace_flag_number }  How do I know what Trace Flags are turned on at the moment?  – You can use the DBCC TRACESTATUS command  Numbers from -1 to seems to be valid *2  Some Trace Flags only works in combination with others  Some Trace Flags only works when used a certain way  Startup, QUERYTRACEON, Global  Turning on all Trace Flags – Do try this at home! (Not at work..) *3

Ok, ok! Now show me the Trace Flags!  Statistics  Locks  Memory  Performance  Troubleshooting  Other

Statistics  2371  Updates statistics more often on tables with more than rows  4136 *4  Uses density vector instead of histogram for cardinality estimate  2389 *4  Enable auto quick statistics update on known ascending columns  2390 *4  Enable auto quick statistics update on all columns

Locks  1200 *5  Prints the process ID and type of lock requested as every request for a lock is made  1204,1222  Prints deadlock info in the errorlog  1211  Disables lock escalation based on memory pressure  1224 *5  Disables lock escalation based on number of locks  1236  Fixes performance problem in scenarios with high SH-DATABASE lock activity in SQL 2012 and SQL 2014

Memory  834  Large Page Allocations (Might be on by default from SQL 2014)  2335  Generates Query Plans optimized for less memory  To be used if you have 512 GB ram or more  2544  Produce a full memory dump in case of an exception  8032  Reverts the cache limit parameters to the SQL Server 2005 RTM setting which in general allows caches such as the Plan Cache to be larger

Performance  610  Minimally logged inserts to indexed tables  661  Disable the ghost record removal process  662  Prints detailed information about the work done by the ghost cleanup task when it runs next. (Use with TF 3605)  1117  Simultaneous Autogrowth in Multiple-file database  1118  Force Uniform Extent Allocation  2312  Forces the query optimizer to use the SQL Server 2014 version of the cardinality estimator  2340  Disable specific SORT optimization in Query Plan  4199  Turn on all optimizations. But from SQL 2016 on, MS has an altered strategy. New major versions will have all ”old” 4199 fixes enabled.  8780  Give the optimizer more time to find a better plan

Troubleshooting  902  Bypass Upgrade Scripts  3607  Skip recovery on startup  3608  Recover only Master db at startup  3609  Do not create tempdb at startup  4022  Bypass Startup procedures

Other  445 – ”Compile Issued” statement to errorlog (with 3605) – New! *6  2388 – Alter DBCC SHOW_STATISTICS output – New!  2529 – Displays DBCC memory allocations – New! *7  Redirect DBCC command output to query window  Directs the output of some Trace Flags to the Errorlog  Turns off ”Backup Successful” messages in errorlog

What Trace Flags should I always turn on?  On new servers: 834 for DW servers, 1117, 1118, 3226, 4199 (not on SQL 2016)  On exsisting servers: 834 for DW servers,1117, 1118, 3226

Read more!  SQL Service Trace Flag List: trace-flag-list/ trace-flag-list/  SQL Server Central TF List:  MSDN TF list:  Albert van der Sel TF list:  Technet Wiki TF list: server.aspx server.aspx  Amit Banerjee TF list: trace-flags/ trace-flags/  Paul Randal discussing TF Pro’s and Con’s: pros-and-cons-of-trace-flags/ pros-and-cons-of-trace-flags/  Some trace flags needs to be specified with ”t” rather than with ”T” in startup options!

Read even more!  QUERYTRACEON us/kb/ https://support.microsoft.com/en- us/kb/