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Creating Universal Product Code version A in Visual Basic .NET Microsoft SQL Server 2008 Internals

Microsoft SQL Server 2008 Internals
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When page compression is rst enabled for a table or partition, SQL Server goes through every full page to determine the possible space savings. (Any pages that are not full are not considered for compression.) This compression analysis actually creates the anchor record, modi es all the columns to re ect the anchor values, and generates the dictionary. Then it compresses each row. If the new compressed page can hold at least ve more rows, or 25 percent more rows than the current page (whichever is larger), then the compressed page replaces the uncompressed page. If compressing the page does not result in this much savings, the compressed page is discarded. When determining what values to use for the anchor record on a compressed page, SQL Server needs to look at every byte in every row, one column at a time. As it scans the column, it also keeps track of possible dictionary entries (which can be used in multiple columns). The anchor record values can be determined for each column in a single pass; that is, by the time all the bytes in all the rows for the rst column are examined once, SQL Server has determined the anchor record value for that column or it has determined that no anchor record value will save suf cient space. As SQL Server examines each column, it collects a list of possible dictionary entries. As we ve discussed, the dictionary contains values that occur enough times on the page so that replacing them with a symbol is cost-effective in terms of space. For each possible dictionary entry, SQL Server keeps track of the value, its size, and the count of occurrences. If (size_of_data_value 1) * (count 1) 2 is greater than zero, it means the dictionary replacement saves space, and the value is considered eligible for the dictionary. In general, SQL Server tries to keep no more than 300 entries in the dictionary, so if more dictionary entries are possible, they are sorted by count during the analysis and only the most frequently occurring values are used in the dictionary.
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Rebuilding the CI Record
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If a table is enabled for either page or row compression, new rows are always compressed before they are inserted into the table. However, the CI record containing the anchor record and the dictionary is rebuilt on an all-or-nothing basis; that is, SQL Server does not just add some new entry to the dictionary when new rows are inserted. SQL Server evaluates whether to rebuild the CI record when the page has been changed a suf cient number of times. It keeps track of changes to each page in the PageModCount eld of the CI record, and that value is updated every time a row is inserted, updated, or deleted. If a full page is encountered during a data modi cation operation, SQL Server examines the PageModCount value. If the PageModCount value is greater than 25, or if the value PageModCount/<number of rows on the page> is greater than 25 percent, SQL Server applies the compression analysis as it does when it rst compresses a page. Only if it is determined that recompressing the page makes room for at least ve more rows (or 25 percent more rows than the current page) does the new compressed page replace the old page. There are some important differences between compression of pages in a B-tree and compression of pages in a heap.
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Compression of B-tree Pages For B-trees, only the leaf level is page-compressed. When inserting a new row into a B-tree, if the compressed row ts on the page, it is inserted, and nothing more is done. If it doesn t t, SQL Server tries to recompress the page, according to the conditions described in the preceding section. If the recompression succeeded, it means that the CI record changed, so the new row must be recompressed and then SQL Server tries to insert it into the page. Again, if it ts, it is simply inserted. If the new compressed row doesn t t on the page, even after possibly recompressing the page, the page needs to be split. When splitting a compressed page, the CI record is copied to a new page, exactly as is, except that the PageModCount value is set to 25. This means that the rst time the page gets full, it gets a full analysis to determine if it should be recompressed. B-tree pages are also checked for possible recompression during index rebuilds (either online or of ine) and during shrink operations. Compression of Heap Pages Pages in a heap are checked for possible compression only during rebuild and shrink operations. (Note that SQL Server 2008 provides an option to rebuild a table and specify a compression level for just this reason.) Also, if you drop a clustered index on a table so that it becomes a heap, SQL Server runs compression analysis on any full pages. To make sure that the RowID values stay the same, heaps are not recompressed during normal data modi cation operations. Although the PageModCount value is maintained, SQL Server never tries to recompress a page based on the PageModCount value.
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