asp.net c# qr code generator FIGURE 6-9 Execution plan for the ad hoc paging solution in C#

Generate QR Code ISO/IEC18004 in C# FIGURE 6-9 Execution plan for the ad hoc paging solution

FIGURE 6-9 Execution plan for the ad hoc paging solution
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Subqueries, Table Expressions, and Ranking Functions
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The gure shows only the left part of the plan starting with the Sequence Project, which assigns the row numbers. If you look at the properties of the Top operator, you can see that the plan scans only the rst 10 rows of the table. Because the code requests the second page of ve rows, only the rst two pages are scanned. Then the Filter operator lters only the second page (rows 6 through 10). Another way to demonstrate that the whole table is not scanned is by populating the table with a large number of rows and running the query with the SET STATISTICS IO option turned on. You will notice by the number of reads reported that when you request page n, regardless of the size of the table, only the rst n pages of rows are scanned. This solution can perform well even when you have multiple page requests that usually move forward that is, page 1 is requested, then page 2, then page 3, and so on, as long as a small number of pages is requested and you have an index to support the requests. When the rst page of rows is requested, the relevant data/index pages are physically scanned and loaded into cache (if they re not there already). When the second page of rows is requested, the data pages for the rst request already reside in cache, and only the data pages for the second page of rows need to be physically scanned. This requires mostly logical reads (reads from cache), and physical reads are needed only for the requested page. Logical reads are much faster than physical reads, but keep in mind that they also have a cost that accumulates. Multipage Access Another solution for paging typically performs better overall than the previous solution when you have multiple page requests that do not move forward, if the result set is not very large. First, materialize all pages in a table along with row numbers and create a clustered index on the row number column:
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SELECT ROW_NUMBER() OVER(ORDER BY qty, empid) AS rownum, empid, mgrid, qty INTO #SalesRN FROM dbo.Sales; CREATE UNIQUE CLUSTERED INDEX idx_rn ON #SalesRN(rownum);
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Now you can satisfy any page request with a query like the following:
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DECLARE @pagesize AS INT, @pagenum AS INT; SET @pagesize = 5; SET @pagenum = 2; SELECT rownum, empid, mgrid, qty FROM #SalesRN WHERE rownum BETWEEN @pagesize * (@pagenum-1) + 1 AND @pagesize * @pagenum ORDER BY rownum;
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The execution plan for this query is shown in Figure 6-10 (abbreviated by removing the operators that calculate boundaries up to the Merge Interval operator to focus on the actual data access).
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FIGURE 6-10 Execution plan for multipaging solution
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This is a very ef cient plan that performs a seek within the index to reach the low boundary row (row number 6 in this case), followed by a partial scan (not visible in the plan), until it reaches the high boundary row (row number 10). Only the rows of the requested page of results are scanned within the index. If your application design is such that it disconnects after each request, obviously the temporary table will be gone as soon as the creating session disconnects. In such a case, you might want to create a permanent table that is logically temporary. You can achieve this by naming the table some_name<some_identi er> for example, T<guid> (Global Unique Identi er). You also need to develop a garbage-collection (cleanup) process that gets rid of tables that the application didn t have a chance to drop explicitly in cases where it terminated in a disorderly way. In cases where you need to support large result sets or a high level of concurrency, you will have scalability issues related to tempdb resources. You can develop a partitioned solution that materializes only a certain number of pages and not all of them for example, 1,000 rows at a time. Typically, users don t request more than the rst few pages anyway. If a user ends up requesting pages beyond the rst batch, you can materialize the next partition (that is, the next 1,000 rows). When you don t care about materializing the result set in a temporary table for multipage access, you might want to consider using a table variable to hold the rst batch of pages (for example, 1,000 rows). Table variables don t involve recompilations, and they suffer less from logging and locking issues. The optimizer doesn t maintain distribution statistics for table variables, so you should be very cautious and selective in choosing the cases to use them for. But when all you need to do is store a small result set and scan it entirely anyway, this technique is ne. Once you re done using this table, you can drop it:
DROP TABLE #SalesRN;
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