SQL Index Fragmentation?


I have some fragmented indexes in SQL 2008 R2.

I have been reindexing them with:


or command like:

alter index ... on dbo... REBUILD

anyway not all indexes seem to reduce in fragmentation to 0.

So I am curious have I got an unreasonable expectation here?

what should I expect the outcome to be? And why might some
not reduce to 0?

btw here is the SQL I am running to work out fragmentation:

declare @current_time as nvarchar(20)

set @current_time = convert(varchar, getdate(), 3) + ' ' + convert(varchar, getdate(), 24)

SELECT @current_time as Timestamp,
DB_NAME(database_id) AS [Database Name], OBJECT_NAME(ps.OBJECT_ID) AS [Object Name],
i.name AS [Index Name], ps.index_id, index_type_desc,
avg_fragmentation_in_percent, fragment_count, page_count
FROM sys.dm_db_index_physical_stats(DB_ID(),NULL, NULL, NULL ,'LIMITED') AS ps
ON ps.[object_id] = i.[object_id] AND ps.index_id = i.index_id
WHERE database_id = DB_ID()
AND page_count > 1500
ORDER BY OBJECT_NAME(ps.OBJECT_ID), avg_fragmentation_in_percent DESC OPTION (RECOMPILE);


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Vitor MontalvãoMSSQL Senior EngineerCommented:
Your script it's only taking in consideration the page count (1500+) but Microsoft's recommendation is to check for fragmentation level. Here's the MSDN article about it where you'll see that a reindex should be performed when fragmentation level between 5% and 30% and a rebuild should be performed when fragmentation's higher than 30%.
whorsfallAuthor Commented:

Thanks for that so I can assume after a rebuild then the fragmentation should be 0?

Plus assuming I get all the tables by changing my query.

And what about the command dbcc dbreindex?


Vitor MontalvãoMSSQL Senior EngineerCommented:
You can't assume a fragmentation level of 0%. That depends if there's no records changes (INSERT, UPDATE and DELETE) during or immediately after the reindex. But it's almost sure that you can assume a fragmentation lower than 5%.

Don't use DBCC REINDEX because will be deprecated in the next SQL Server version. Use ALTER INDEX instead, with REORGANIZE option for avg_fragmentation_in_percent between 5% and 30% or with REBUILD option for avg_fragmentation_in_percent > 30%.

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whorsfallAuthor Commented:

Thanks for the clarification that was great :)

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Microsoft SQL Server 2008

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