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Category: Trace Flags

Confirming Checkpoints

Published 2016-08-25 by Kevin Feasel

Arun Sirpal shows how to log when checkpointing runs:

Via Configuration manager I enabled trace flags 3502 and 3605 – both needed to get the checkpoint information and write it to the error log.

I then shutdown the machine, on start-up I looked into the error log.

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EXEC XP_READERRORLOG

Notice the ‘s’ in front of the spid<number>? Well that means the checkpoint was done via the automatic process; if you do a manual checkpoint it won’t see this letter.

I did not know that the “s” indicated that this was an automated process.

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New Columnstore Trace Flags

Published 2016-07-29 by Kevin Feasel

Niko Neugebauer looks at a few trace flags which modify columnstore index behavior:

Starting with SQL Server 2016, if you have enough RAM and suffering from the TempDB Spills that do have a significant impact on your workload, then you can enable the Trace Flag 9389 that will enable Batch Mode Iterators to request additional memory for the work and thus avoiding producing additional unnecessary I/O.

I am glad that Microsoft has created this functionality and especially that at the current release, it is hidden behind this track, and so Microsoft can learn from the applications before enabling it by default, hopefully in the next major release of SQL Server.

There’s a lot of good stuff in here.  Read the whole thing.

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Trace Flag 2389

Published 2016-07-25 by Kevin Feasel

Erin Stellato looks at using Trace Flag 2389 with the new cardinality estimator in SQL Server 2014:

To summarize, when using compatibility mode 110 or below, trace flag 2389 works like it always has.  But when using compatibility mode 120 or higher, and thus the new CE, the estimates are not the same compared to the old CE, and in this specific case, are not that different whether using the trace flag or not.

So what should you do?  Test, as always.  I haven’t found anything documented in MSDN that states that trace flag 2389 is not supported with compatibility mode 120 and higher, nor have I found anything that documents a change in behavior.  I do find it very interesting that the estimates are different (in this case much lower) with the new CE.  That could potentially be an issue, but there are multiple factors in play when it comes to estimates, and this was a very simple query (one table, one predicate).  In this case, the estimate is way off (4920 rows versus the 22,595 rows for the June 5 date).

I highly recommend reading this article.

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Cardinality Estimator Trace Flags

Published 2016-06-21 by Kevin Feasel

Kim Tripp shows how to set which cardinality estimator you want to use on a per-query basis:

However, the bad news is the QUERYTRACEON is limited to SysAdmin only (be sure to read the UPDATEs at the end of this post). Jack Li (Microsoft CSS) wrote a great article about a problem they solved by using a logon trigger to change the CE for an entire session: Wanting your non-sysadmin users to enable certain trace flags without changing your app? Now, I do want to caution you that setting master to trustworthy is not something you should take lightly. But, you should NOT let anyone other than SysAdmin have any other rights in master (other than the occasional EXEC on an added user-defined SP). Here are a couple of posts to help warn you of the danger:

A warning about the TRUSTWORTHY database option
Guidelines for using the TRUSTWORTHY database setting in SQL Server

Read on for a couple of options.

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Are Trace Flags Dead?

Published 2016-03-10 by Kevin Feasel

Brent Ozar notes that several common trace flags are getting turned into all-grown-up commands:

SQL Server 2016 does away with these unintuitive trace flags by adding new ALTER DATABASE commands:

ALTER DATABASE SET MIXED_PAGE_ALLOCATION ON (or OFF, which is the new default behavior)

ALTER DATABASE MODIFY FILEGROUP [myfilegroup] AUTOGROW_ALL_FILES (or AUTOGROW_SINGLE_FILE, which is still the default)

I think trace flags will still be around for quite some time as a troubleshooting mechanism, but I certainly prefer clearer naming (was that trace flag 1117…or 1171…or maybe…).

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Trace Flag 834 And Columnstore

Published 2015-12-15 by Kevin Feasel

Chris Bell warns us against having Trace Flag 834 turned on in an instance which contains columnstore indexes:

[I]t is not recommended to have trace flag 834 on when using columnstore indexes in your databases.

Since the 834 trace flag is a global level flag, and columnstores are in individual databases I wrote the script below to go through and check if you ave any columnstore indexes, and then check if the trace flag is enabled.

Chris also has a helpful script to see if your instance has this issue.

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