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We ve seen how SQL Server stores data and index information. Now we look at what SQL Server actually does internally when your data is modi ed. We ve seen how clustered indexes de ne logical order to your data and how a heap is nothing more than a collection of unordered pages. We ve seen how nonclustered indexes are structures stored separately from the data and how that data is a copy of the actual table s data, de ned by the index de nition. And, as a rule of thumb, you should always have a clustered index on a table. The SQL Customer Advisory Team published a white paper in mid-2007 that compares various table structures and essentially supports this view; see http://www.microsoft.com/ technet/prodtechnol/sql/bestpractice/clusivsh.mspx. In this section, we review how SQL Server deals with the existence of indexes when processing data modi cation statements. Note that for every INSERT, UPDATE, and DELETE operation on a table, the equivalent operation also happens to every nonclustered index on the table. The mechanisms described in this section apply equally to clustered and nonclustered indexes. Any modi cations to the table are made to the heap or clustered index rst, then to each nonclustered index in turn. In SQL Server 2008, the exception to this rule is ltered indexes, where the lter predicate means the ltered nonclustered index may not have a matching row for the table row being modi ed. When changes are made to the table, the ltered index predicate is evaluated to determine whether it is necessary to apply the same operation to the ltered nonclustered index.
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When inserting a new row into a table, SQL Server must determine where to put the data, as well as insert a corresponding row into each nonclustered index. Each operation follows the same pattern: modify the appropriate data page (based on whether or not the table has a clustered index) and then insert the corresponding index rows into the leaf level of each nonclustered index. When a table has no clustered index that is, when the table is a heap a new row is always inserted wherever room is available in the table. In 3, you learned how IAMs keep track of which extents in a le already belong to a table and in 5, you saw how the PFS pages indicate which of the pages in those extents have available space. If no pages with space are available, SQL Server tries to nd unallocated pages from existing uniform extents
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that already belong to the object. If none exists, SQL Server must allocate a whole new extent to the table. 3 discussed how the Global Allocated Maps (GAMs) and Shared Global Allocation Maps (SGAMs) are used to nd extents available to be allocated to an object. So, although locating space in which to do an INSERT is relatively ef cient using the PFS and IAM, because the location of a row (on INSERT) is not de ned, determining where to place a row within a heap is usually less ef cient than if the table has a clustered index. For an INSERT into a table with a clustered index and for index rows being inserted into nonclustered indexes, the row (regardless of whether it s a data row or an index row) always has a speci c location within the index where it must be inserted, based on the value the new row has for the index key columns. An INSERT occurs either when the new row is the direct result of an INSERT or when it s the result of an UPDATE statement that either causes the row to move or for an index key column to change. When a row has to move to a new page, the UPDATE statement is internally executed using a DELETE followed by an INSERT (the DELETE/ INSERT strategy). New rows are inserted based on their index key position, and SQL Server splices in a new page via a page split if the current leaf level (a data page if this is the clustered index or an index page if this is a nonclustered index) has no room. Because the index dictates a particular ordering for the rows in the leaf level of the index, every new row has a speci c location where it belongs. If there s no room for the new row on the page where it belongs, a new page must be allocated and linked into the B-tree. If possible, this new page is allocated from the same extent as the other pages to which it is linked. If the extent is already full (which is usually the case), a new extent (eight pages or 64 KB) is allocated to the object. As described in 3, SQL Server uses the GAM pages to nd an available extent.
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