MCITP SQL Server 2005 Database Developer All-in-One Exam Guide in Visual Studio .NET

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MCITP SQL Server 2005 Database Developer All-in-One Exam Guide
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change the fill factor to 80 percent so it would take longer for a page split to occur But what if page splits still occur with the fill factor set to 80 percent Maybe we should then change the fill factor to something lower, say 50 percent or 20 percent Of course, this brings up another question Why don t we just set the fill factor to a low number, such as 20 percent for all the indexes The answer is because there s a maintenance cost Just as page splits impact performance, leaving pages empty makes the indexes larger An index with a fill factor of 50 percent makes the index twice as large TIP The fill factor becomes a significant factor in the ultimate size of the index Consider an index that s 20MB with a fill factor of 100 percent How big would the index be if we set the fill factor at 50 percent, or allowed the pages to be half full It would be twice as large Part of your goal when determining the fill factor is striking a balance between (1) having the pages as full as possible and (2) having the least amount of page splits One of the ways to determine how many page splits are occurring is by measuring fragmentation Fragmentation A side effect of page splits is fragmentation of the index The more page splits you have, the more fragmented an index becomes We can see this in Figure 7-14
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Extent A 1 2 3 4 5 6 7 8 9 Extent B
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Contiguous pages Extent A 1 3 4 6 8 9 12 13 2 Extent B 5 7 10 11 14
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Fragmented index
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Figure 7-14 A fragmented index
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When the index is first created, the pages are created one right after another (page 1, page 2, page 3, and so on) within extents As each page split occurs, many of the pages are moved to other extents, and the pages are no longer in order Suppose we had to read all the data in the fragmented index shown in Figure 7-14 While the data is in only two extents, to be in order, we d have to switch between extents as we reached the end of almost every page We d start in page 1 in Extent A, then switch to Extent B for page 2, then switch back to Extent A for page 3 Switching extents can be resource-intensive if done constantly Later, we ll talk about monitoring indexes for fragmentation and what can be done to fix it once it occurs But if you want to prevent fragmentation, use a fill factor appropriate for the number of inserts you expect so you can prevent, or at least minimize, page splits
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7: Optimizing Databases
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Pad Index
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In Figure 7-11, we showed how the index actually has multiple levels The leaf level of the index is what will be split the most However, for a very heavily modified database, it s also possible that the root level or intermediate levels of the index are being split If we want to specify a fill factor for the nonleaf levels of the index, we specify the pad index option The pad index option directs the system to use the same percentage as the fill factor when filling the nonleaf levels of the index
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Calculating Size
One of the key factors to consider when designing an index is how much space it will take To calculate this, you must consider many factors:
Fill factors The number of columns in the index The fixed data size The number of variable-length columns The average variable-length data size The estimated number of rows Clustered index keys
I realize the preceding bulleted list may look daunting, but really only three concepts are present, and they can be calculated with this simple formula: (S R) / F
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