SQL Server Query Hint–USE PLAN N

on March 22, 2013

The USE PLAN query hint tells the Query Optimizer that you want a new plan, and that the new plan should match the shape of the plan in the supplied XML plan.
This is very similar to the use of plan guides (covered in the next section), but whereas plan guides don’t require a change to the query, the USE PLAN query hint does require a change to the T-SQL being submitted to the server.
Sometimes this query hint is used to solve deadlock issues or other data-related problems. However, in nearly all cases the correct course of action is to address the underlying issue, but that often involves architectural changes, or code changes that require extensive development and test work to get into production. In these cases the USE PLAN query hint can provide a quick workaround for the DBA to keep the system running while the root cause of a problem is found and fixed.
Note that the preceding course of action assumes you have a “good” XML plan from the problem query that doesn’t show the problem behavior. If you just happened to capture a bunch of XML plans from all the queries running on your system when it was working well, then you are good to go, but that’s not typically something that anyone ever does, as you usually leave systems alone when they are working OK; and capturing XML plans for every query running today just in case you may want to use the USE PLAN query hint at some point in the future is not a very useful practice.
What you may be able to do, however, is configure a test system with data such that the plan your target query generates is of the desired shape, capture the XML for the plan, and use that XML plan to “fix” the plan’s shape on your production server.
Before using any query hints, run a web search for the latest information on issues with query hints. Try searching on the keywords “SQL Server Query Hints” and look specifically for anything by Craig Freedman, who has written several great blog entries on some of the issues you can encounter when using query hints.

Related Posts

Leave a Reply