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Statement-Level Recompilation

The next thing we'll look at is a performance enhancement that is part of the infrastructural improvements in T-SQL: statement recompilation. In SQL Server 2000, the query plan architecture differs from previous versions, and it is divided into two structures: a compiled plan and an executable plan.

  • Compiled plan (a.k.a. query plan)—A read-only data structure used by any number of users. The plan is reentrant, which implies that all users share the plan and no user context information (such as data variable values) is stored in the compiled plan. There are never more than one or two copies of the query plan in memory—one copy for all serial executions and another for all parallel executions.

  • Executable plan—A data structure for each user that concurrently executes the query. This data structure, which is called the executable plan or execution context, holds the data specific to each user's execution, such as parameter values.

This architecture, paired with the fact that the execution context is reused, has very much improved the execution of not only stored procedures but functions, batches, dynamic queries, and so on. However, there is a common problem with executing stored procedures, and that is recompilation. Examples of things that cause recompilation to occur are as follows:

  • Schema changes

  • Threshold changes in rows

  • Certain SET options

Figure 02Figure 7-2: Trace Properties Dialog for SQL Profiler

A recompilation can incur a huge cost especially if the procedure, function, or batch is large, because SQL Server 2000 does module-level recompilation. In other words, the whole procedure is recompiled even if the cause of the recompilation affects only a small portion of the procedure. In addition, if the recompilation happens because a SET option changes, the executable plan will be invalidated and not cached. The code in Listing 7-1 is extremely simple, but it can be used to illustrate the problem.

Figure 3Figure 7-3: Trace Output

Listing 7-1 is a stored procedure which in the middle of the procedure changes the CONCAT_NULL_YIELDS_NULL option. When this runs against SQL Server 2000, a recompilation happens for each execution of the procedure.

Listing 7-1: Procedure That Causes Recompilation


SELECT 'before set option'

—//change a set option

SELECT 'after set option'

To verify that recompilation happens on SQL Server 2000, do the following:

  1. Catalog the procedure in Listing 7-1.

  2. Open the SQL Server Profiler and from the File menu, select New | Trace.

  3. When the Trace Properties dialog comes up, choose the Events tab.

  4. In the Stored Procedures event group, choose the SP:Recompile event, click the Add button, as shown in Figure 7-2, and then click Run.

  5. Execute the procedure a couple of times from Query Analyzer and view the trace output.

  6. The output from the trace will show a couple of entries in the Event Class column with the value of SP:Recompile, as in Figure 7-3. This indicates that the procedure has been recompiled.

Figure 4Figure 7-4: Trace Properties Dialog in SQL Server 2005

As mentioned before, the cost of recompilation can be very high for large procedures, and in the SQL Server 2005 release, Microsoft has changed the model to statement-level re-compilation. At this stage you may worry that performance will suffer if each statement in a procedure is individually recompiled. Rest assured that the initial compilation is still on the module level, so only if a recompile is needed is it done per statement.

Another performance benefit in SQL Server 2005 is the fact that when statement recompilation is done, the execution context will not be invalidated. The procedure in Listing 7-1 can be used in SQL Server 2005 to compare the differences between SQL Server 2000 and 2005. In SQL Server 2005, follow the steps listed earlier and notice in the trace how a recompile happens only the first time; for each subsequent execution, there is no recompile. This is due to the fact that an execution plan will be created after the initial recompile. Run the following code after you have executed the procedure a couple of times, and notice that the result you get consists of both a compiled plan and an executable plan.

SELECT * FROM syscacheobjects
WHERE dbid = db_id('pubs') 
AND objid = object_id('test2')

To be certain that you get the correct result, you can clean out the cache before you execute the procedure by executing dbcc freeproccache.

When setting up the trace, you will see how the SQL Profiler allows you to trace more events than in SQL Server 2000. Figure 7-4 shows the Events Selection tab from the Trace Properties dialog.

As mentioned in the beginning of this chapter, the statement-level recompilation can be seen as a purely infrastructural enhancement. As a developer or DBA, you will not explicitly use it even though you implicitly benefit from it, and it may change the way you develop stored procedures. No longer do recompiles have as much of a negative impact on performance.

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