Monitoring and Troubleshooting SQL Server Performance in .NET framework

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Monitoring and Troubleshooting SQL Server Performance
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The query statistics DMVs and DMFs encompass the entire group of functionality related to executing a query in SQL Server. This functionality is broken into two distinct groups: connections to the instance and queries executing inside the engine. Connection information is contained in two DMVs: sys.dm_exec_requests and sys.dm_exec_sessions. Each connection to a SQL Server instance is assigned a system process ID (SPID), with information about each session available in sys.dm_exec_sessions. You can retrieve session information regarding the user or application creating the connection, login time, connection method, and a variety of information concerning the high-level statistics for the state of the connection.
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In previous versions of SQL Server, you would retrieve the information that sys.dm_exec_sessions provides by executing the sp_who or sp_who2 system stored procedures, or by retrieving rows from the sysprocesses table. However, sys.dm_exec_sessions contains significantly more information than previous versions of SQL Server logged.
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Each session in SQL Server will normally be executing a single request. However, it is possible for a single SPID to spawn multiple requests. You can retrieve statistics about each executing request from sys.dm_exec_requests. The requests DMV forms the basis for resolving many performance issues. The information contained within this view can be separated into four categories: query settings, query execution, transactions, and resource allocation. Query settings encompass the options that can be applied to each request executed, such as quoted identifiers, American National Standards Institute (ANSI) nulls, arithabort, transaction isolation level, and so on. Query execution encompasses items such as the memory handle to the SQL statement, the memory handle to the query plan, CPU time, reads, writes, the ID of the scheduler, the SPID blocking the request if applicable, and so on. Transactions encompass such items as the transaction ID, the number of open transactions, the number of result sets, the deadlock priority, and related statistics. Resource allocation encompasses the wait type and wait time.
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The DBA s friend: sys.dm_exec_requests DMV
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Because the sys.dm_exec_requests view is used to determine many different operation states, it will become an extremely familiar tool for any DBA managing a SQL Server server.
Lesson 4: Using Dynamic Management Views and Functions
Detailed query statistics are contained within the sys.dm_exec_query_stats and sys.dm_exec_cached_plans DMVs. Query stats provides detailed statistics related to the performance of a query as well as the amount of resources the query consumed. Using this DMV, you can determine the number of reads (logical and physical), writes (logical and physical), CPU, and elapsed time for a query. The DMV tracks these statistics based on the SQL handle and also contains the plan handle.
MORE INFO
Query plans, execution plans, and the query optimizer
Every SQL statement that is executed must be compiled. After it is compiled, it is stored in the query cache and identified by a memory pointer called a handle. The SQL Server query optimizer then must determine a query plan for the statement. After the query plan is determined, it is also stored in the query cache and identified by a memory pointer. The compiled plan then generates an execution plan for the query to use. When the query executes, the sys.dm_exec_query_stats DMV tracks the SQL handle with the associated plan handle for that execution, as well as all the statistical information for that query. The details of query plans, execution plans, and the query optimizer are beyond the scope of this book, but you can find comprehensive coverage of these topics in the book Inside SQL Server 2005: The Storage Engine, by Kalen Delaney (Microsoft Press, 2007).
You use the sys.dm_exec_cached_plans DMV, which is similar to syscacheobjects in previous SQL Server versions, to retrieve information about query plans. SQL Server query plans can be of two basic types: compiled and execution. A compile plan is generated for each unique SQL statement that has been executed. Parameters and literals are substituted with generic placeholders so that execution of a stored procedure with varying values for parameters, for example, is still treated as the same SQL statement and does not cause the optimizer to create additional plans. Compiled plans are reentrant, meaning that they can be reused. An execution plan, on the other hand, is created for each concurrent execution of a particular statement. Thus, if 15 connections were executing the same stored procedure concurrently, regardless of whether the parameters were the same, there would be one compiled plan and 15 execution plans in the query cache. Although the SQL handle and the plan handle are meaningful to the SQL Server engine, they are meaningless to a person. So SQL Server provides two functions to translate the information. The sys.dm_exec_sql_text DMF takes a single parameter of the SQL handle and returns in text format the query that was executed. The sys.dm_exec_query_plans DMF takes a single parameter of the plan handle and returns an XML showplan.
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