SQL Server 2008 Trace Flag 4135

Question

Does anyone have any idea what this flag does? There is no reference to it on MSDN. It is not a part of the article that lists available trace flags in 2008.

There is a KB article that recommends that it is turned on to resolve a specific issue with SQL Server 2008 that we have run into, but we would like to at least know what this flag does before enabling it.

http://support.microsoft.com/kb/960770

Answer 1

I managed to get an answer to this with the help of Jonathan Kehayias on the MSDN forums.

The traceflag disables an optimization in the query optimizer. The optimization caused the issue described in the KB article when you try to insert into a table by selecting from the table itself. As turning on the traceflag could result in a perf degradation, you only should use it if you run into the issue described in the KB article.

The thread discussing this is here.

Answer 2

At a guess, I'm thinking this turns on the hotfix included in Cumulative Update 3 should your system encounter the specific issue described in the knowledge base article.