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  SQL Server Tips by Robin Schumacher

Forwarded Rows

SQL Server will move rows in a table under certain conditions. One situation might arise when a row in a table that has a variable-length column is updated to a larger size that will no longer fit on its original page. When SQL Server creates a forwarding pointer, the pointer remains in place unless one of two things happens. The first is when a row shrinks enough to move back to its original location. The second is when the entire database shrinks. When a database file shrinks, SQL Server will reassign the row identifiers, which are used as the row locators, so the shrink process never generates forwarded rows.

At times, forwarded records can reduce performance because additional I/O is involved to first obtain the record pointer to the relocated row, and then the row itself. But, when does row forwarding become a problem? For example, just because a table has one hundred forwarded rows, does that mean a performance problem exists? It depends. If the table has one hundred rows in it, a problem does exist because 100% of the table suffers from forwarded rows. If the table has three million rows, the forwarded row situation involving one hundred rows is likely not causing much fuss in the database.

The fragmentation diagnostic procedures supplied earlier in this chapter will help the DBA identify tables with forwarded row problems. The same holds true for the new SQL Server 2005 sys.dm_db_index_physical_stats function. In particular, the procedures provide both a forwarded record count and a forwarded record percent so tables that could benefit from being reorganized can be easily pinpointed.

In terms of removing forwarded rows, the options include reorganizing the table or performing a shrink of a database or database file.


The above book excerpt is from:

High-Performance SQL Server DBA
Tuning & Optimization Secrets

ISBN: 0-9761573-6-5
Robin Schumacher

 http://www.rampant-books.com/book_2005_2_sql_server_dba.htm  

 

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