Plan
forcing was introduced in SQL Server 2005 to provide a way for you to
supply an entire execution plan that SQL Server will use to execute a
query. Plan forcing is useful when the query optimizer will not produce
an optimal execution plan for a query, even though you know a better
execution plan is available. If you have a query that performed better
prior to an upgrade, you may be able to capture the old execution plan
and force the optimizer to use it. You can supply the execution plan in
XML format following a query by supplying the USE PLAN option, as shown in Listing 1.
Example 1. Syntax Used to Specify an Execution Plan by Supplying the USE PLAN Query Hint
SELECT *
FROM Table1
JOIN Table2
ON Table1.Column = Table2.Column
OPTION (USE PLAN 'N
<ShowPlanXML xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan">
...
</ShowPlanXML>')
|
In order to supply the
execution plan, you must first capture it in XML format. There are a few
ways you can capture the XML execution plan to supply the query.
You can use the SHOWPLAN_XML SET statement to return the XML execution plan instead of running the query, as shown in the following code:
SET SHOWPLAN_XML ON
GO
SELECT * FROM Table
SET SHOWPLAN_XML OFF
GO
You can use the STATISTICS XML SET statement to return the XML execution plan following the query result set, as shown in the following code:
SET STATISTICS XML ON
GO
SELECT * FROM Table
SET STATISTICS XML OFF
GO
You can use the query_plan column in the sys.dm_exec_query_plan Dynamic Management Function, as shown in the following query:
SELECT est.text, eqp.query_plan
FROM sys.dm_exec_cached_plans ecp
CROSS APPLY sys.dm_exec_query_plan(ecp.plan_handle) eqp
CROSS APPLY sys.dm_exec_sql_text(ecp.plan_handle) est
You can use the Showplan XML, Showplan XML Statistics Profile, and Showplan XML For Query Compile event classes in SQL Server Profiler.
You can use the Display Estimated Execution Plan and Include Actual Execution Plan
options in SQL Server Management Studio; right-click the execution
plan, and select Show Execution Plan XML from the context menu.
If the XML execution plan is
invalid in any way, the query will fail. Certain underlying database
schema changes may cause the execution plan to become invalid.
Therefore, it is extremely important to test any queries that have the USE PLAN
query hint after making database changes. Just as with any option you
use to override the query optimizer, you should apply the USE PLAN
query hint with extreme caution, and then only after you have exhausted
your other options, such as creating proper indexes and updating
statistics. Since the USE PLAN query
hint forces a query to utilize a specified execution plan, the query
optimizer will no longer be able to dynamically adjust to changes within
the database.
