asp.net c# qr code generator Inside Microsoft SQL Server 2008: T-SQL Querying in C#.NET

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Inside Microsoft SQL Server 2008: T-SQL Querying
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1 + ABS(CHECKSUM(NEWID())) % (20*60), starttime) AS endtime INTO dbo.BigSessions FROM ( SELECT 'app' + CAST(1 + ABS(CHECKSUM(NEWID())) % 10 AS VARCHAR(10)) AS app, 'user1' AS usr, 'host1' AS host, DATEADD( second, 1 + ABS(CHECKSUM(NEWID())) % (30*24*60*60), '20090101') AS starttime FROM dbo.Nums WHERE n <= 1000000 ) AS D; CREATE UNIQUE CLUSTERED INDEX idx_ucl_keycol ON dbo.BigSessions(keycol); CREATE INDEX idx_nc_app_st_et ON dbo.BigSessions(app, starttime, endtime);
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I populated the table with sessions that start at random times over a period of one month and last up to 20 minutes. I also distributed 10 different application names randomly. Now request an estimated execution plan for the original query, and you will get the plan shown in Figure 4-66.
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FIGURE 4-66 Estimated execution plan for query against the BigSessions table with adequate sample data
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Query Tuning
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The cost of the query is now 52,727. Trust me: You don t want to run it to see how long it really takes. Or, if you like, you can start running it and come back the next day hoping that it nished. Now that the sample data is more realistic, you can see that the set-based solution presented in this section is slow unlike what you might be led to believe when using inadequate sample data. In short, you can see how vital it is to put some thought into preparing good sample data. Of course, the tuning process only starts now; you might want to consider query revisions, cursor-based solutions, revisiting the model, and so on. But here I wanted to focus the discussion on bad sample data. I ll conduct a more thorough tuning discussion related to the problem at hand in Inside T-SQL Programming.
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TABLESAMPLE
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SQL Server supports a feature that allows you to sample data from an existing table. The tool is a clause called TABLESAMPLE that you specify after the table name in the FROM clause along with some options. Here s an example for using TABLESAMPLE to request 1,000 rows from the Orders table in the Performance database:
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SELECT * FROM dbo.Orders TABLESAMPLE (1000 ROWS);
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Note that if you run this query you probably won t get exactly 1,000 rows. I ll explain why shortly. You can specify TABLESAMPLE on a table-by-table basis. Following the TABLESAMPLE keyword, you can optionally specify the sampling method to use. Currently, SQL Server supports only the SYSTEM method, which is also the default if no method is speci ed. In the future, we might see additional algorithms. Per ANSI, the SYSTEM keyword represents an implementation-dependent sampling method. This means you will nd different algorithms implemented in different products when using the SYSTEM method. In SQL Server, the SYSTEM method implements the same sampling algorithm used to sample pages to generate distribution statistics. You can use either the ROWS or the PERCENT keyword to specify how many rows you would like to get back. Based on your inputs, SQL Server calculates random values to gure out whether a page should be returned. Note that the decision of whether to read a portion of data is done at the page level. This fact, along with the fashion in which SQL Server determines whether to pick a page based on a random factor, means that you won t necessarily get the exact number of rows that you asked for; rather, you ll get a fairly close value. The more rows you request, the more likely you are to get a result set size close to what you requested.
Inside Microsoft SQL Server 2008: T-SQL Querying
Here s an example for using the TABLESAMPLE clause in a query against the Orders table, requesting 1,000 rows:
SELECT * FROM dbo.Orders TABLESAMPLE SYSTEM (1000 ROWS);
I ran this query three times and got a different number of rows every time: 880, 1200, and 920. An important bene t you get with the SYSTEM sampling method is that only the chosen pages (those that SQL Server picked) are scanned. So even if you query a huge table, you will get the results pretty fast as long as you specify a fairly small number of rows. As I mentioned earlier, you can also specify a percentage of rows. Here s an example requesting 0.1 percent, which is equivalent to 1,000 rows in our table:
SELECT * FROM dbo.Orders TABLESAMPLE (0.1 PERCENT);
When you use the ROWS option, SQL Server internally rst converts the speci ed number of rows to a percentage. Remember that you are not guaranteed to get the exact number of rows that you requested; rather, you ll get a close value determined by the number of pages that were picked and the number of rows on those pages (which may vary). To make it more likely that you ll get the exact number of rows you are after, specify a higher number of rows in the TABLESAMPLE clause and use the TOP option to limit the upper bound that you will get, like so:
SELECT TOP (1000) * FROM dbo.Orders TABLESAMPLE (2000 ROWS);
There s still a chance that you will get fewer rows than the number you requested, but you re guaranteed not to get more. By specifying a higher value in the TABLESAMPLE clause, you increase the likelihood of getting the number of rows you are after. If you need to get repeatable results, use a clause called REPEATABLE, which was designed for this purpose, providing it with the same seed in all invocations. For example, running the following query multiple times yields the same result, provided that the data in the table has not changed:
SELECT * FROM dbo.Orders TABLESAMPLE (1000 ROWS) REPEATABLE(42);
Note that with small tables you might not get any rows at all. For example, run the following query multiple times, requesting a single row from the Production.ProductCostHistory table in the AdventureWorks2008 database:
SELECT * FROM AdventureWorks2008.Production.ProductCostHistory TABLESAMPLE (1 ROWS);
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