asp.net c# qr code generator 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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Run the query again and notice how the number of logical reads goes back to over 4,000. Note that the key list is limited to 16 columns and 900 bytes. An added bonus with included nonkey columns is that they are not bound by the same limitations. In fact, they can even include large objects such as variable-length columns de ned with the MAX speci er and XML columns.
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Index Intersection
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So far, I ve focused mainly on the performance bene t you get from indexes when reading data. Keep in mind, though, that indexes incur a cost when you modify data. Any change of data (deletes, inserts, updates) must be re ected in the indexes that hold a copy of that data, and it might cause page splits and adjustments in the balanced trees, which can be very expensive. Therefore, you cannot freely create as many indexes as you like, especially in systems that involve intensive modi cations, such as OLTP environments. You want to prioritize and pick the more important indexes. This is especially a problem with covering indexes because different queries can bene t from completely different covering indexes, and you might end up with a very large number of indexes that your queries could bene t from. Fortunately, the problem is somewhat reduced because the optimizer supports a technique called index intersection, where it intersects data obtained from two indexes and, if required, then intersects the result with data obtained from another index and so on. For example, the optimizer will use index intersection for the following query, producing the plan shown in Figure 4-47:
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SELECT orderid, custid FROM dbo.Orders WHERE shipperid = 'A';
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FIGURE 4-47 Execution plan with index intersection
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I will elaborate on join operators in 7. The optimal index here would be one where shipperid is de ned as the key column and orderid and custid are de ned as included nonkey columns but no such index is on the table. Rather, the index idx_nc_sid_od_i_cid de nes the
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Query Tuning
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shipperid as the key column and also contains the custid column, and the index PK_Orders contains the orderid column. The optimizer used the access method nonclustered index seek + ordered partial scan to obtain the relevant data from idx_nc_sid_od_i_cid, and it used an unordered nonclustered index scan to obtain the relevant data from PK_Orders. It then intersected the two sets based on the row locator values; naturally, row locator values pointing to the same rows will be matched. You can think of index intersection as an internal join based on a match in row locator values. Here are the performance measures that I got for this query:
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Scan count 6 Logical reads 3771 Physical reads 84 Read-ahead reads 672 CPU time 1248 ms Elapsed time 4357 ms Estimated subtree cost 13.0864
Filtered Indexes and Statistics
SQL Server 2008 introduces support for ltered indexes and statistics. A ltered index is an index on a subset of rows de ned based on a predicate. Filtered indexes are cheaper to create and to maintain compared to non ltered ones because only modi cations to the relevant subset of rows need to be re ected in the index. Also, ltered distribution statistics (histograms) whether created on the rst index key column or otherwise are more accurate than non ltered statistics. That s because the maximum number of steps in a histogram is limited, and with ltered statistics that number is used to represent a smaller set of rows. I ll provide several scenarios in which you may nd ltered indexes useful. The rst scenario involves queries that lter data based on a column that has a large percentage of NULLs. When ltering rows based on a predicate in the form <column> <operator> <value>, the lter eliminates rows with a NULL in that column. The optimizer is well aware of this fact. Therefore, if you create an index on this column excluding rows where the column is NULL, the optimizer will still consider using the index for such predicates. The following example demonstrates this capability. Run the following code to create an index on the Sales.SalesOrderHeader table in the AdventureWorks2008 database with CurrencyRateID as the key and a lter based on the predicate CurrencyRateID IS NOT NULL:
USE AdventureWorks2008; CREATE NONCLUSTERED INDEX idx_currate_notnull ON Sales.SalesOrderHeader(CurrencyRateID) WHERE CurrencyRateID IS NOT NULL;
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