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Determining which indexes can be useful for finding the desired rows is just like indexing for SELECT operations, which I've discussed in several previous columns, so I won't cover that again.Instead, let's look at what happens during the DELETE and UPDATE operations.If you then update the lastname value to Zappa, SQL Server has to move the row to near the end of the table.When a row moves because of a change in the clustered-index key value, SQL Server must update every nonclustered index on the table to reflect the new value.Because SQL Server deletes a row from all nonclustered indexes whenever you delete a row from a table, one deleted row can result in many ghost records.SQL Server doesn't "ghost" rows in non-leaf index pages when they're deleted, but as with heap pages, SQL Server doesn't compress the space until new index rows need space on the page.The nonclustered indexes don't need changing; they still refer to the original location, and from there, the forwarding pointer directs SQL Server to the new location.

For example, if you have a clustered index on the lastname column, SQL Server will store a row with a lastname value of Abbot near the beginning of the table.When the last row is deleted from a data page, SQL Server deallocates the entire page.(Unless the page is the only one remaining in the table—a table always contains at least one page, even if it's empty.) This deallocation also results in the deletion from the index page of the row that pointed to the old data page.Compaction doesn't occur until a page needs additional contiguous space for inserting a new row.Second, in the index's leaf level, when rows are deleted, SQL Server marks them as ghost records.When you're deciding which columns to build your clustered index on, remember that the nonclustered index pointers use the clustered-index key as a bookmark for locating the data row. If a row moves because it no longer fits on the original page, it still has the same row locator (i.e., the clustering key for the row stays the same), and SQL Server doesn't have to modify any nonclustered indexes.