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These dynamic management objects make the analysis of index usage simple and accurate.
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I referred to index fragmentation on multiple occasions in this chapter. When I mentioned fragmentation, I referred to a type known as logical scan fragmentation or average fragmentation in percent or external fragmentation. As I mentioned earlier, this type re ects the percentage of out-of-order pages in the index in terms of their le order versus their logical order in the linked list. Remember that this fragmentation can have a substantial impact on ordered scan operations in indexes. It has no effect on operations that do not rely on the index s linked list for example, seek operations, lookups, allocation order scans, and so on.
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You want to minimize the fragmentation level of indexes for queries with a substantial portion of their cost involved with ordered scans. You do so by rebuilding or reorganizing indexes. Another type of fragmentation that you typically care about is what I referred to as average page density. Some database professionals refer to this type of fragmentation as internal fragmentation, but to avoid confusion I consciously didn t use this term earlier. Although logical scan fragmentation is never a good thing, average page density has two facets. A low percentage (low level of page population) has a negative impact on queries that read data because they end up reading more pages than they could potentially if the pages were better populated. The positive impact of having some free space in index pages is that insertions of rows to such pages would not cause page splits, which are very expensive. As you can guess, free space in index pages is bad in systems that involve mostly reads (for example, data warehouses) and good for systems that involve many inserts (for example, OLTP systems). You might even want to introduce some free space in index pages by specifying a llfactor value when you rebuild your indexes. To determine whether you need to rebuild or reorganize your indexes, you need information about both types of fragmentation. You can get this information by querying the DMF dm_db_index_physical_stats. For example, the following query will return fragmentation information about the indexes in the Performance database:
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SELECT * FROM sys.dm_db_index_physical_stats( DB_ID('Performance'), NULL, NULL, NULL, 'SAMPLED');
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The fragmentation types I mentioned show up in the attributes avg_fragmentation_in_ percent and avg_page_space_used_in_percent, and as you can see, the attribute names are self-explanatory. As I mentioned earlier, to treat both types of fragmentation you need to rebuild or reorganize the index. Rebuilding an index has the optimal defragmentation effect. The operation makes its best attempt to rebuild the index such that the le order of the pages is as close as possible to their order in the linked list and to make the pages as contiguous as possible. Also, remember that you can specify a llfactor to introduce some free space in the index leaf pages. Note that if your computer has multiple CPUs and SQL Server uses parallel index rebuilds (Enterprise edition only), the operation will nish faster than with a single thread but is likely to result in more logical fragmentation. You can restrict the operation to a single CPU with the MAXDOP hint this way, at the cost of a longer index rebuild, you will likely get less fragmentation. Also, SQL Server needs space for sorting in the legroup where the index resides. If the legroup les have only a little free space, some logical fragmentation in the index at the end of the operation is likely. To minimize fragmentation, ensure that you have suf cient free space in the les or use the option SORT_IN_TEMPDB to request that the index rebuild use space from the tempdb database for sorting. By default, index rebuilds are of ine operations. Rebuilding a clustered index acquires an exclusive lock for the whole duration of the operation, meaning that other processes can
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