upc czech internet Short Data Region Long Data Region Special Information in Visual Basic .NET

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Short Data Region Long Data Region Special Information
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Header
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CD Region
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FIGURE 7-7 General structure of a CD record
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I ll describe each of these sections in detail. Header The row header is always a single byte and roughly corresponds to what I called Status Bits A in 5. The bits have the following meanings: Bit 0 Bit 1 Indicates the type of record; it s 1 for the new CD record format. Indicates that the row contains versioning information.
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Bits 2 through 4 Taken as a three-bit value, these bits indicate what kind of information is stored in the row. The possible values are the following:
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000 primary record 001 ghost empty record 010 forwarding record 011 ghost data record 100 forwarded record 101 ghost forwarded record 110 index record 111 ghost index record
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7
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Bit 5
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Indicates that the row contains a long data region (with values greater than 8 bytes in length). Not used in SQL Server 2008.
Bit 6 - 7
The CD Region The CD region is composed of two parts. The rst part is either 1 or 2 bytes, indicating the number of short columns. If the most signi cant bit of the rst byte is set to 0, then it is a 1-byte eld with a maximum value of 127. If there are more than 127 columns, then the most signi cant bit is 1, and SQL Server uses 2 bytes to represent the number of columns, which can be up to 32,767. Following the 1 or 2 bytes for the number of columns is the CD array. The CD array uses four bits for each column in the table, to represent information about the length of the column. Four bits can have 16 different possible values, but in SQL Server 2008, only 13 of them are used:
0 (0x0) indicates that the corresponding column is NULL 1 (0x1) indicates that the corresponding column is a 0-byte short value. 2 (0x2) indicates that the corresponding column is a 1-byte short value. 3 (0x3) indicates that the corresponding column is a 2-byte short value. 4 (0x4) indicates that the corresponding column is a 3-byte short value. 5 (0x5) indicates that the corresponding column is a 4-byte short value. 6 (0x6) indicates that the corresponding column is a 5-byte short value. 7 (0x7) indicates that the corresponding column is a 6-byte short value. 8 (0x8) indicates that the corresponding column is a 7-byte short value. 9 (0x9) indicates that the corresponding column is an 8-byte short value. 10 (0xa) indicates that the corresponding column is long data value and uses no space in the short data region. 11 (0xb) is used for columns of type bit with the value of 1. The corresponding column takes no space in the short data region. 12 (0xc) indicates that the corresponding column is a 1-byte symbol, representing a value in the page dictionary. (I ll talk about the dictionary in the section entitled Page Compression, later in this chapter).
The Short Data Region The short data region doesn t need to store the length of each of the short data values because that information is available in the CD region. However, if there are hundreds of columns in the table, it can be expensive to access the last columns. To minimize this cost, columns are grouped into clusters of 30 columns each and at the beginning of the short data region, there is an area called the short data cluster array. Each array entry is a
Microsoft SQL Server 2008 Internals
single-byte integer and indicates the sum of the sizes of all the data in the previous cluster in the short data region, so that the value is basically a pointer to the rst column of the cluster. The rst cluster of short data starts right after the cluster array, so no cluster offset is needed for it. There may not be 30 data columns in a cluster, however, because only columns with a length less than or equal to 8 bytes are stored in the short data region. As an example, consider a row with 64 columns, and columns 5, 10, 15, 20, 25, 30, 40, 50, and 60 are long data, and the others are short. The CD region contains the following:
A single byte containing 64, the number of columns, in the CD region. A CD array of 4 * 64 bits, or 32 bytes, containing information about the length of each column. There are 55 entries with values indicating an actual data length for the short data, and 8 entries of 0xa, indicating long data. The short data region contains the following.
A short data cluster offset array containing the two values, each containing the length of a short data cluster. In this example, the rst cluster, which is all the short data in the rst 30 columns, has a length of 92, so the 92 in the offset array indicates that the second cluster starts 92 bytes after the rst. The number of clusters can be calculated as (Number of columns 1) /30. The maximum value for any entry in the cluster array is 240, if all 30 columns were short data of 8 bytes in length. All the short data values.
Figure 7-8 illustrates the CD region and the short data region with sample data for the row described previously. The CD array is shown in its entirety, with a symbol indicating the length of each of the 64 values. So the array can t on a page of this book, the actual data values are not shown. The rst cluster has 24 values in the short data region (6 are long values), the second cluster has 27 (3 are long) and the third cluster has the remaining 4 columns (all short). I ll discuss the storage of the long values next.
CD Region Number of columns N = 64 CD array --64 4-bit values ( a indicates long column) 3285a4358a6543a3456a6666a5463a 254372644a745269277a463495736a 5433 Short Data Region Length of short data in each 30column cluster (N 1)/30 values 92 106 Three clusters of actual data 24 27 values values 4 values
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