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Reorganizing an index is the only change that doesn t have a corresponding option in the CREATE INDEX command. The reason for this is that when you create an index, there is nothing to reorganize. The REORGANIZE option replaces the DBCC INDEXDEFRAG command and removes some of the fragmentation from an index, but it is not guaranteed to remove all the fragmentation, just as DBCC INDEXDEFRAG may not remove all the fragmentation (in spite of its name). Before we discuss removing fragmentation, we must rst discuss detecting fragmentation, which we do in the next section.
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As we ve already seen in numerous examples, the output of sys.dm_db_index_physical_stats returns a row for each level of an index. However, when a table is partitioned, it effectively treats each partition as a table, so this DMV actually returns a row for each level of each partition of each index. For a small index with only in-row data (no row-over ow or LOB pages) and only the one default partition, we might get only two or three rows back (one for each index level). But if there are multiple partitions and additional allocation units for the row-over ow and LOB data, we might see many more rows. For example, a clustered index on a table containing row-over ow data, built on 11 partitions and being two levels deep, have 33 rows (2 levels 11 partitions + 11 partitions for the row_over ow allocation units) in the fragmentation report returned by sys.dm_db_index_ physical_stats. The section entitled Tools for Analyzing Indexes, earlier in this chapter, has a comprehensive discussion of the input parameters and the output results, but the following columns give fragmentation information that is not obvious:
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Forwarded_record_count Forwarded records (discussed in the section entitled Data Modi cation Internals, earlier in this chapter) are possible only in a heap and occur when a row with variable-length columns increases in size due to updates so that it no longer ts in its original location. If a table has lots of forwarded records, scanning the table can be very inef cient. Ghost_Record_Count and version_ghost_record_count Ghost records are rows that physically still exist on a page but logically have been removed, as discussed in the section entitled Data Modi cation Internals. Background processes in SQL Server clean up ghost records, but until that happens, no new records can be inserted in their place. So if there are lots of ghost records, your table has the drawback of lots of internal fragmentation (that is, the table is spread out over more pages and takes longer to scan) with none of the advantages (there is no room on the pages to insert new rows to avoid external fragmentation). A subset of ghost records is measured by version_ghost_record_count. This value reports the number of rows that have been retained by an outstanding Snapshot isolation transaction. These are not cleaned up until all relevant transactions have been committed or rolled back. Snapshot isolation is discussed in 10.
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If fragmentation becomes too severe and is affecting query performance, you have several options for removing it. You might also wonder how severe is too severe. First of all, fragmentation is not always a bad thing. The biggest performance penalty from having fragmented data arises when your application needs to perform an ordered scan on the data. The more the logical order differs from the physical order, the greater the cost of scanning the data. If, on the other hand, your application needs only one or a few rows of data, it doesn t matter whether the table or index data is in logical order or is physically contiguous, or whether it is spread all over the disk in totally random locations. If you have a good index to nd the rows you are interested in, SQL Server can nd one or a few rows very ef ciently, wherever they happen to be physically located. If you are doing ordered scans of an index (such as table scans on a table with a clustered index, or a leaf-level scan of a nonclustered index), it is frequently recommended that if your avg_fragmentation_in_percent value is between 5 and 20, you should reorganize your index to remove the fragmentation. As we see shortly, reorganizing an index (also called defragging) compacts the leaf-level pages back to their originally speci ed llfactor and then rearranges the pages in the leaf level to correct the logical fragmentation, using the same pages that the index originally occupied. No new pages are allocated, so this is a much more space-ef cient operation than rebuilding the index. If the avg_fragmentation_in_percent value is greater than 30, you should consider completely rebuilding your index. Rebuilding an index means that a whole new set of pages is allocated for the index. This removes almost all fragmentation, but it is not guaranteed to eliminate it completely. If the free space in the database is itself fragmented, you might not be able to allocate enough contiguous space to remove all gaps between extents. In addition, if other work is going on that needs to allocate new extents while your index is being rebuilt, the extents allocated to the two processes can end up being interleaved. Defragmentation is designed to remove logical fragmentation from the leaf level of an index while keeping the index online and as available as possible. When defragmenting an index, SQL Server acquires an Intent-Exclusive lock on the index B-tree. Exclusive page locks are taken on individual pages only while those pages are being manipulated, as we see later in this chapter when we describe the defragmentation algorithm. Defragmentation in SQL Server 2008 is initiated using the ALTER INDEX command. The general form of the command to remove fragmentation is as follows:
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ALTER INDEX { index_name | ALL } ON <object> REORGANIZE [ PARTITION = partition_number ] [ WITH ( LOB_COMPACTION = { ON | OFF } ) ]
ALTER INDEX with the REORGANIZE option offers enhanced functionality compared to DBCC INDEXDEFRAG in SQL Server 2000. It supports partitioned indexes, so you can choose to defragment just one particular partition (the default is to defragment all the partitions), and it allows you to control whether the LOB data is affected by the defragmenting.
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