qr code generator in vb.net Figure 3-46. Execution plan for query against indexed view in .NET framework

Generate QR Code in .NET framework Figure 3-46. Execution plan for query against indexed view

Figure 3-46. Execution plan for query against indexed view
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The view contains a very small number of rows (around a couple of thousands) compared to the number of rows in the table (a million). The leaf of the index contains only about 10 pages. Hence,
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the I/O cost of the plan would be about 10 page reads. Here are the performance measures I got for this query: Logical reads: 10 CPU time: 0 ms Elapsed time: 78 ms Estimated subtree cost: 0.011149 Interestingly, if you work with an Enterprise (or Developer) edition of SQL Server, the optimizer will consider using indexes on the view even when querying the underlying tables directly. For example, the following query produces a similar plan to the one shown in Figure 3-46 , with the same query cost: SELECT empid, YEAR(orderdate) AS orderyear, COUNT_BIG(*) AS numorders FROM dbo.Orders GROUP BY empid, YEAR(orderdate);
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If you're not working with an Enterprise edition, you have to query the view directly, and also specify that you do not want the optimizer to expand its optimization choices beyond the scope of the view. You do so by specifying the NOEXPAND table hint: FROM <view_name> WITH (NOEXPAND) . SQL Server 2005 enhances the circumstances in which indexed views are used, supporting subintervals, logically equivalent filter expressions, and more. As I mentioned, I'll point you to resources that describe these in detail.
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Index Optimization Scale
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Recall the earlier discussion about the tuning methodology. When you perform index tuning, you do so with respect to the query patterns that incur the highest cumulative costs in the system. For a given query pattern, you can build an index optimization scale that would help you make the right design choices. I will demonstrate this process through an example. To follow the demonstrations, before you continue, drop the view created earlier and all the indexes on the Orders table, except for the clustered index. Alternatively, you can rerun the code in Listing 3-1 , after commenting or removing all index and primary key creation statements on Orders, keeping only the clustered index. In our example, suppose that you need to tune the following query pattern: SELECT orderid, custid, empid, shipperid, orderdate FROM dbo.Orders WHERE orderid >= value;
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Remember that the efficiency of some access methods depends on the selectivity of the query, while the efficiency of others doesn't. For access methods that depend on selectivity, assume that the query pattern is typically fairly selective (around 0.1 percent selectivity, or around 1000 qualifying rows). Use the following query in your tuning process when aiming at such selectivity: SELECT orderid, custid, empid, shipperid, orderdate FROM dbo.Orders WHERE orderid >= 999001;
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I'll progress in the index optimization scale from the worst-case scenario to the best, using this query as a reference, but I'll also describe what would happen when the selectivity of the query changes.
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Table Scan (Unordered Clustered Index Scan)
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The worst-case scenario for our query pattern with fairly high selectivity is when you have no good index. You will get the execution plan shown in Figure 3-47 , using a table scan (unordered clustered index scan).
Figure 3-47. Execution plan with table scan (unordered clustered index scan)
Even though you're after a fairly small number of rows (1000 in our case), the whole table is scanned. I got the following performance measures for this query: Logical reads: 25080 CPU time: 1472 ms Elapsed time: 16399 Estimated subtree cost: 19.6211 This plan is trivial and not dependent on selectivitythat is, you would get the same plan regardless of the selectivity of the query.
Unordered Covering Nonclustered Index Scan
The next step in the optimization scale would be to create a covering nonclustered index where the filtered column (orderid ) is not the first index column: CREATE NONCLUSTERED INDEX idx_nc_od_i_oid_cid_eid_sid ON dbo.Orders(orderdate) INCLUDE(orderid, custid, empid, shipperid);
This index would yield an access method that uses a full unordered scan of the leaf of the index as shown in Figure 3-48 :
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