upc internet pl Deleting Rows in the Non-Leaf Levels of an Index in Visual Basic .NET

Encoder UPC A in Visual Basic .NET Deleting Rows in the Non-Leaf Levels of an Index

Deleting Rows in the Non-Leaf Levels of an Index
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When you delete a row from a table, all nonclustered indexes must be maintained because every nonclustered index has a pointer to the row that s now gone. Rows in index non-leaf pages aren t ghosted when deleted, but just as with heap pages, the space isn t compacted until new index rows need space in that page.
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When the last row is deleted from a data page, the entire page is deallocated by the ghost cleanup background thread. The exception is if the table is a heap, as we discussed earlier. (If the page is the only one remaining in the table, it isn t deallocated. A table always contains at least one page, even if it s empty.) Deallocation of a data page results in the deletion of the row in the index page that pointed to the deallocated data page. Non-leaf index pages are deallocated if an index row is deleted (which, again, for an update might occur as part of a DELETE/INSERT strategy), leaving only one entry in the index page. That entry is moved to its neighboring page if there is space, and then the empty page is deallocated. The discussion so far has focused on the page manipulation necessary for deleting a single row. If multiple rows are deleted in a single DELETE operation, you must be aware of some other issues. Because the issues of modifying multiple rows in a single query are the same for INSERTs, UPDATEs, and DELETEs, we discuss this issue in its own section, later in this chapter.
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SQL Server updates rows in multiple ways, automatically and invisibly choosing the fastest update strategy for the speci c operation. In determining the strategy, SQL Server evaluates the number of rows affected, how the rows are accessed (via a scan or an index retrieval, and via which index), and whether changes to the index keys occur. Updates can happen either in place, by just changing one column s value to a new value in the original row, or as a DELETE followed by an INSERT. In addition, updates can be managed by the query processor or by the storage engine. In this section, we examine only whether the update happens in place or whether SQL Server treats it as two separate operations: delete the old row and insert a new row. The question of whether the update is controlled by the query processor or the storage engine is actually relevant to all data modi cation operations (not just updates), so we look at that in a separate section.
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What happens if a table row has to move to a new location In SQL Server 2008, this can happen because a row with variable-length columns is updated to a new, larger size so that it no longer ts on the original page. It can also happen when the clustered or nonclustered index column(s) change because rows are logically ordered by the index key. For example, if we have a clustered index on lastname, a row with a lastname value of Abbot is stored near the beginning of the table. If the lastname value is then updated to Zappa, this row has to move to near the end of the table. Earlier in this chapter, we looked at the structure of indexes and saw that the leaf level of nonclustered indexes contains a row locator, or bookmark, for every single row in the table. If the table has a clustered index, that row locator is the clustering key for that row. So if and only if the clustered index key is being updated, modi cations are required in every nonclustered index (with the possible exception of ltered nonclustered indexes). Keep this in mind when you decide on which columns to build your clustered index. It s a great idea to cluster on a nonvolatile column, such as an identity. If a row moves because it no longer ts on the original page, it still has the same row locator (in other words, the clustering key for the row stays the same), and no nonclustered indexes have to be modi ed. This is true even if the table is moved to a new physical location ( legroup or partitioning scheme). Nonclustered indexes are updated only if the clustering key changes, and moving the physical location of a table row does not change its clustering key. In our discussion of index internals, you also saw that if a table has no clustered index (in other words, if it s a heap), the row locator stored in the nonclustered index is actually the physical location of the row. In SQL Server 2008, if a row in a heap moves to a new page, the row leaves a forwarding pointer in the original location. The nonclustered indexes won t need to be changed; they still refer to the original location, and from there, they are directed to the new location. In this case, if the table moves to a new location ( legroup or partitioning scheme), the nonclustered indexes are updated, as the physical location of all records in the heap must change, thus invalidating the prior row locators in the nonclustered indexes. Let s look at an example. We have created a table a lot like the one we created for doing inserts, but this table has a third column of variable length. After we populate the table with ve rows, which ll the page, we update one of the rows to make its third column much longer. The row no longer ts on the original page and has to move. We can then load the output from DBCC IND into the sp_tablepages table to get the page numbers used by the table:
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USE AdventureWorks2008; GO DROP TABLE bigrows; GO CREATE TABLE bigrows ( a int IDENTITY , b varchar(1600), c varchar(1600)); GO
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