c# rdlc barcode font Figure 7-8 A page split between two extents in VS .NET

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Figure 7-8 A page split between two extents
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Page splits occur whenever a page fills up If we anticipate many inserts that would cause page splits, we would create the index with a fill factor less than 100 EXAM 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 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 7-9
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Figure 7-9 A fragmented index
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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 5 7 Extent B 10 11 14
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When the index is first created, the pages are created one right after another (page 1, page 2, page 3, etc) 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 7-9 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
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But if you want to prevent fragmentation, use a fill factor appropriate for the number of inserts you expect so that you can prevent, or at least minimize, page splits
Pad Index
In Figure 7-6 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
Calculating Size
One of the key factors to consider when designing an index is how much space it will take To calculate this, you need to consider many factors:
Fill factor Number of columns in the index Fixed data size Number of variable-length columns Average variable-length data size Estimated number of rows Clustered index key
I realize the preceding bulleted list may look daunting, but there are only three concepts here, and they can be calculated with this simple formula: (S R) / F
S = Size of index row R = number of rows in index F = Fill factor
Size of Index Row
To determine the size of an index row, we identify the size of the columns in the index If our index is just the PRIMARY KEY displayed as an integer, it is 4 bytes long That s the size of an index row If we used a composite index of last name and first name, it would be a little different The LastName column may be defined as an nvarchar(50), and the FirstName column may be defined as a nvarchar(35) But as nvarchar, it only uses the space it needs We can easily calculate the average length used by any nvarchar column by using the AVG and LEN functions The following script will calculate the average length of both the FirstName and LastName columns in the PersonContact table in AdventureWorks:
7: Optimizing Databases
USE AdventureWorks SELECT AVG(Len(FirstName)) FROM PersonContact GO SELECT AVG(Len(LastName)) FROM PersonContact
This script shows us the average length of the first name is 5 characters and the average length of the last name is 5 characters Since these are stored as nvarchar (instead of varchar), they take twice as much space so that the characters can be displayed in different languages Our index row size (S) is 20 bytes This is calculated as (5 2) for the first name + (5 2) for the last name
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