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If all you are going to do is read data within a cursor, make certain that you are declaring the cursor as read-only. Read-only cursors require less overhead than updatable cursors.

Access to rows within a cursor can be restricted by using the FORWARD_ONLY and SCROLL options. If a cursor is declared as FORWARD_ONLY, each row can be read only once as the cursor pointer advances through the result set. If you declare a cursor using the SCROLL option, the FETCH statement has the following options:

FETCH NEXT Fetches the next row in the result set based on the current position of the pointer. FETCH NEXT is equivalent to just executing FETCH, which also moves forward one row at a time within the cursor result set. FETCH PRIOR

Fetches the row in the result set just before the current position of the Fetches the nth row from the beginning of the result set.

Fetches the nth row forward in the cursor result set from the current position of the cursor pointer.

T-SQL has three concurrency options available for cursors:

READ_ONLY SQL Server does not acquire a lock on the underlying row in the table because a cursor marked as READ_ONLY cannot be updated. SCROLL_LOCKS A lock is acquired as each row is read into the cursor, guaranteeing that any transaction executed against the cursor succeeds. OPTIMISTIC

A lock is not acquired. SQL Server instead uses either a timestamp or a calculated checksum in the event that a timestamp column does not exist to detect if the data has changed since being read into the cursor. If the data has changed, the modification fails.

When a stored procedure is created, SQL Server checks the syntax but does not validate any of the objects referenced within the procedure. The first time you execute a stored procedure, SQL Server parses and compiles the code. When the procedure is compiled for the first time, a check is made to ensure that all objects referenced either exist or will be created within the procedure prior to being accessed. At compilation time, a query plan is generated and stored in the query cache. This compile plan is reentrant and is reused each time the procedure is executed. Each concurrent execution of a stored procedure also generates a query plan for execution, which is called the execution plan. Execution plans are also stored in the query cache, but they are non-reentrant. Once a connection has finished executing the procedure, the execution plan stored in the query cache can be assigned to the next connection that executes the procedure. Therefore, a single stored procedure that is heavily used can have a single compile plan along with many execution plans in the query cache, each of which consumes memory. Under most circumstances, storing the compile plan for reuse eliminates the resources that need to be used to generate a query plan each time the procedure is executed. However,

162 CHAPTER 5 Programming Microsoft SQL Server with T-SQL User-Defined Stored Procedures, Functions, Triggers, and Views

if the stored procedure contains multiple code paths depending on the results of condition checking, dramatically different query plans could be generated for the procedure. By reusing the same query plan for each execution, performance can suffer in some cases. If a stored procedure generates a different query plan the majority of the time that it is executed, you should create the procedure using the RECOMPILE option. When the RECOMPILE option is enabled for a stored procedure, SQL Server does not cache and reuse a query plan. In addition to the RECOMPILE option, SQL Server can detect if statistics are out of date during the execution of a procedure. If SQL Server determines that the query plan could be less than optimal during the execution of a procedure, execution will stop while a new query plan is generated for the next statement to execute within the procedure. Prior to SQL Server 2005, the query plan for the entire stored procedure would be regenerated. However, since SQL Server 2005, recompilation occurs at the statement level.