Microsoft SQL Server 2012 Internals

Hotfixes

The SQL Server engineering team is extremely careful about making changes to the Query Optimizer when those changes could lead to plan quality regressions. Unfortunately, changing the Optimizer’s model can often cause unintentional side-effects because almost every customer uses this portion of the product. The SQL Server team has been working on delivering features that minimize the risk of regression for customers, especially in production environments. As part of this effort, the team has been formalizing the way it delivers hotfixes for releases of SQL Server after the initial release of each major version.

Starting several years ago, most optimizer hotfixes are controlled in a new way. You must turn on a special traceflag (4199) that ...