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The execution plan is then stored in the procedure cache and executed from there. Different steps in the execution plan will be posted to different modules of the relational engine to be executed: DML manager, DDL manager, stored procedure manager, transaction manager, or utility manager. Results are collected in the form of a resultset and sent to the user.
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Execution plans remain in the procedure cache for a while. If the same or some other user issues a similar batch, the relational engine will first attempt to find a matching execution plan in the procedure cache. If it exists, it will be reused. If it does not exist, Microsoft SQL Server will parse and compile a batch.
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A simple query can be reused only in two scenarios. First, the query text of the second query must be identical to the text of the query described by the execution plan in the cache. Everything has to match: spaces, line breaks, indentation even case on case-sensitive servers. The second scenario is when the query contains fully qualified database objects to reuse execution plans:
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The designers of SQL Server have created two methods to improve the reuse of queries that are not designed as stored procedures:
Autoparameterization The sp_executesql stored procedure
We will cover the first of these methods in the following section and the second one in 9.
Autoparameterization
When a Transact-SQL statement is sent to SQL Server, it attempts to determine whether any of its constants can be replaced with parameters. Subsequent queries that use the same template will reuse the same execution plan. For example, let s say that SQL Server receives the following ad hoc query:
SELECT FirstName, LastName, Phone, Fax, Email, OrgUnitId, UserName FROM Asset.dbo.Contact where ContactId = 3
It will try to parameterize it in the following manner and create an execution plan:
SELECT FirstName, LastName, Phone, Fax, Email, OrgUnitId, UserName FROM Asset.dbo.Contact where ContactId = @P1
After this, all similar queries will reuse the execution plan:
SELECT FirstName, LastName, Phone, Fax, Email, OrgUnitId, UserName FROM Asset.dbo.Contact where ContactId = 11
SQL Server will apply autoparameterization only when a query s template is safe that is, when the execution plan will not
SQL Server 2000 Stored Procedure Programming
be changed and the performance of SQL Server will not be degraded if parameters are changed.
NOTE: SQL Server might decide to create and use a different execution plan even if the query is based on the same field. For example, imagine that you are querying a table with contact information using the Country field. If your company is operating predominantly in North America, SQL Server might carry out a query for Denmark contacts based on the index on the Country field and a query for USA contacts as a table scan.
SQL Server will attempt autoparameterization on Insert, Update, and Delete statements too. In fact, the query must match a set of four templates in order for SQL Server to attempt autoparameterization:
Select {* | column-list} From table Where column-expression [Order by column-list] Insert table Values ({constant | NULL | Default} [, ...n]) Update table set column-name = constant where column-expression Delete table Where column-expression
Note that a column-expression is an expression that involves only column names, constants, the And operator, and comparison operators: <, >, =, >=, <=, and <>. SQL Server is more forgiving about formatting the query when autoparameterization is used, but it still does not allow changes in capitalization or changes in the way an object is qualified.
3:
Stored Procedure Design Concepts
Reuse of Stored Procedure Execution Plans
Stored procedures do not have the limitations associated with ad hoc queries, and that is the main reason stored procedures are reused more often then queries. The reuse of execution plans is one of the main reasons why the use of stored procedures is a better solution than the use of ad hoc queries. For example, if you execute a query three times, SQL Server will have to parse, recompile, and execute it three times. A stored procedure will most likely be parsed and recompiled only once just before the first execution.
NOTE: Someone might argue that the time needed to compile is insignificant compared with the time needed to execute a query. That is sometimes true. But the SQL Server query engine in this version compares dozens of new processing techniques in order to select the best one to process the query or stored procedure. Therefore, the time needed to recompile a stored procedure is greater than it used to be.
The execution plan consists of two parts. One is reentrant and can be used concurrently by any number of processes. The other part contains the data context; that is, the parameters to be used during execution. Although this part can be reused, it cannot be used by another process concurrently, so more instances of this part will be created. The execution plan will be removed from the procedure cache when a process called lazywriter concludes that the execution plan has not been used for a while, or when the execution plan s dependent database objects are changed in any of the following ways:
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