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Microsoft SQL Server 2008 Internals
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PageModCount The PageModCount value keeps track of the changes to this particular page and is used when determining whether the compression on the page should be reevaluated, and a new CI record built. I ll talk more about how this value is used in the next section, when I discuss page compression analysis. Offsets The offsets contain values to help SQL Server nd the dictionary. It contains a value indicating the page offset for the end of the anchor record and a value indicating the page offset for the end of the CI record itself. Anchor Record The anchor record looks exactly like a regular CD record on the page, including the record header, the CD array, and both a short data area and a long data area. The values stored in the data area are the common pre x values for each column, some of which might be NULL. Dictionary The dictionary area is composed of three sections. The rst is a 2-byte eld containing a numeric value representing the number of entries in the dictionary. The second section is an offset array of 2-byte entries, indicating the end offset of each dictionary entry relative to the start of the dictionary data section. The third section contains the actual dictionary data entries. Remember that each dictionary entry is a byte string that is replaced in the regular data rows by a symbol. The symbol is simply an integer value from 0 to N. In addition, remember that the byte strings are datatype-independent; that is, they are just bytes. After SQL Server determines what recurring values are stored in the dictionary, it sorts the list rst by data length, then by data value, and then assigns the symbols in order. So suppose that the values to be stored in the dictionary are these:
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0x 0x 0x 0x 0x 53 FF DA 34 12 51 F8 15 F3 34 4C 43 77 64 B6 22 CD 56
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Table 7-8 shows the sorted dictionary, along with the length and symbol for each entry.
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TABLE 7-8
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Values in a Page Compression Dictionary
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Length
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2 bytes 3 bytes 3 bytes 4 bytes 4 bytes
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Value
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0x FF F8 0x 12 34 56 0x 53 51 4C 0x 34 F3 B6 22 CD 0x DA 15 43 77 64
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Symbol
0 1 2 3 4
7
Special Storage
The dictionary area would then look like Figure 7-15.
Header
Offsets
Dictionary
02 00 05 00 08 00 0D 00 12 00
0x FF F8 0x 12 34 56 0x 53 51 4C 0x 34 F3 B6 22 CD 0x DA 15 43 77 64
FIGURE 7-15 The dictionary area in a Compression Information Record
Note that the dictionary never actually stores the symbol values. They are stored only in the data records that need to use the dictionary. Because they are simply integers, they can be used as an index into the offset list to nd the appropriate dictionary replacement value. For example, if a row on the page contains the dictionary symbol [2], SQL Server looks in the offset list for the third entry, which in Figure 7-14 ends at offset 0800 from the start of the dictionary. SQL Server then nds the value that ends at that byte, which is 0x 53 51 4C. If this byte string was stored in a char or varchar column (that is, a single-byte character string), it would correspond to the character string SQL. I illustrated earlier in this chapter that the DBCC PAGE output shows you the CD array for compressed rows. For compressed pages, DBCC PAGE shows the CI record and details about the anchor record within it. In addition, with format 3, DBCC PAGE shows details about the dictionary entries. When I captured the DBCC PAGE in format 3 for the rst page of my Employees_pagecompressed table and copied it to a Microsoft Of ce Word document, it needed 261 pages. Needless to say, I will not show you all that output. Even when I just copied the CI record information, it took 7 pages, which is still too much to show in this book. I ll leave it to you to explore the output of DBCC PAGE for the tables with compressed pages.
Page Compression Analysis
In this section, I discuss some of the details regarding how SQL Server determines whether to compress a page or not and what values it uses for the anchor record and the dictionary. Row compression is always performed when requested, but page compression depends on the amount of space that can be saved. However, the actual work of compressing the rows has to wait until page compression has been performed. Because both types of page compression, pre x substitution and dictionary symbol substitution, replace the actual data values with encodings, the row cannot be compressed until SQL Server determines what encodings are going to replace the actual data.
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