Inside Microsoft SQL Server 2008: T-SQL Querying in Visual C#.NET

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Inside Microsoft SQL Server 2008: T-SQL Querying
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that you want to clear all supported caches. The pool_name value indicates the name of a Resource Governor pool cache that you want to clear. For the undocumented options, specify Object Plans to clear object plans (plans for stored procedures, triggers, and user-de ned functions). Specify SQL Plans to clear plans for ad-hoc statements, including prepared statements. Specify Bound Trees to clear plans for views, constraints, and defaults.
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Caution Consider carefully before using these commands in production environments.
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Obviously, clearing the cache has a performance impact on the system. After clearing the data cache, SQL Server needs to physically read pages accessed for the rst time from disk. After clearing execution plans from the cache, SQL Server needs to generate new execution plans for queries. Also, be sure that you are aware of the impact of clearing the cache even when doing so in development or test environments.
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Dynamic Management Objects
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SQL Server 2005 introduced for the rst time support for dynamic management objects, including DMVs and DMFs. SQL Server 2008 added new objects and in some cases added new attributes to existing objects. These contain extremely useful information about the server that you can use to monitor SQL Server, diagnose problems, and tune performance. Much of the information provided by these views and functions has never before been available. Studying them in detail is time very well spent. In these books, I make use of the ones that are relevant to my discussions, but I urge you to take a close look at others as well. You can nd information about them in SQL Server Books Online.
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STATISTICS IO
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STATISTICS IO is a session option used extensively throughout these books. It returns I/O-related information about the statements that you run. To demonstrate its use, rst clear the data cache:
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DBCC DROPCLEANBUFFERS;
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Then run the following code to turn the session option on and invoke a query:
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SET STATISTICS IO ON; SELECT orderid, custid, empid, shipperid, orderdate, filler FROM dbo.Orders WHERE orderdate >= '20060101' AND orderdate < '20060201';
You should get output similar to the following:
Table 'Orders'. Scan count 1, logical reads 536, physical reads 3, read-ahead reads 548, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
4
Query Tuning
The output tells you how many times the table was accessed in the plan (Scan count); how many reads from cache were involved (logical reads); how many reads from disk were involved ( physical reads and read-ahead reads); and similarly, how many logical and physical reads related to large objects were involved (lob logical reads, lob physical reads, lob read-ahead reads). Run the following code to turn the session option off:
SET STATISTICS IO OFF;
Measuring the Run Time of Queries
STATISTICS TIME is a session option that returns the net CPU and elapsed clock time information about the statements that you run. It returns this information for both the time it took to parse and compile the query and the time it took to execute it. To demonstrate the use of this session option, rst clear both the data and execution plans from cache:
DBCC DROPCLEANBUFFERS; DBCC FREEPROCCACHE;
Run the following code to turn the session option on:
SET STATISTICS TIME ON;
Then invoke the following query:
SELECT orderid, custid, empid, shipperid, orderdate, filler FROM dbo.Orders WHERE orderdate >= '20060101' AND orderdate < '20060201';
You will get output similar to the following:
SQL Server parse and compile time: CPU time = 0 ms, elapsed time = 64 ms. SQL Server parse and compile time: CPU time = 0 ms, elapsed time = 1 ms. SQL Server Execution Times: CPU time = 31 ms, elapsed time = 711 ms.
The output tells you the net CPU time and elapsed clock time for parsing and compiling the query and also the time it took to execute it. Run the following code to turn the option off:
SET STATISTICS TIME OFF;
This tool is convenient when you want to analyze the performance of an individual query interactively. When you run benchmarks in batch mode, the way to measure the run time of queries is different. Store the value of the SYSDATETIME function in a variable directly before the query. Directly after the query, issue an INSERT statement into the table where you collect
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