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After searching the internet i couldn't find the reason for

  1. Why do we need to rebuild and reorganize indexes in SQL Server ?

  2. what does internally happens when we rebuild and reorganize ?

An article on a site says :

Index should be rebuild when index fragmentation is great than 40%. Index should be reorganized when index fragmentation is between 10% to 40%. Index rebuilding process uses more CPU and it locks the database resources. SQL Server development version and Enterprise version has option ONLINE, which can be turned on when Index is rebuilt. ONLINE option will keep index available during the rebuilding.

I couldn't understand this, though it says that WHEN to do this, but I would like to know WHY do we need to rebuild and reorganize indexes ?

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This is the question about why, and here is the question about when. –  Nick Chammas Jul 29 '13 at 8:21
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migrated from stackoverflow.com Nov 9 '12 at 15:22

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1 Answer

up vote 14 down vote accepted

As you perform inserts updates and deletes, your indexes will become fragmented both internally and externally.

Internal fragmentation is you have a high percentage of free space on your index pages, meaning that SQL Server needs to read more pages when scanning the index.

External fragmentation is when the pages of the index are not in order any more, so SQL Server has to do more work, especially in IO terms to read the index.

If your indexes become too fragmented, at best, your queries will be less efficient but at worst, SQL Server will just stop using the indexes all together, meaning virtually all queries would have to perform a table scan or clustered index scan. This will hurt your performance a lot!

When you reorganise an index, then SQL Server uses the existing index pages and just shuffels data around on those ages. This will elliviate internal fragmentation and can also remove a small amount of external fragmentation. It is a lighter weight operation than rebuild and is always online.

When you rebuild an index, SQL Server actually resorts the data of the index and uses a new set of index pages. This will obviously elliviate both internal and external fragmentation but is a more heavy weight operation and by default causes the index to go offline, although it can be performed as an online operation, depending on your SQL Server version and settings.

Please do not expect to have 0 fragmentation after a Rebuild however. Unless you use a MAXDOP query hint, SQL Server will parallelise the rebuild operation and the more processors involved, the more fragmentation there is likely to be, because each processor or core, will rebuild their section or fragment of the index individually, without regard for each other. This is a trade off between best fragmentation levels and time taken to rebuild the index. For near 0 fragmentation, use MAXDOP 1 and sort the results in TempDB

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thank you for your prompt reply, now i understood when to do rebuild and reorganize indexes. also it would better to ReBuild only when the traffic is less or no traffic at all on our website. –  Mourya Nov 9 '12 at 7:35
    
Yes, that is correct. You can actually use server agent to respond to low CPU usage on the server by starting a job. This job could contain your index rebuild scripts. Also, if you are in a 24/7 environment, I suggest you write a script that only reorganises or rebuilds the indexes that actually require it, rather than rebuilding all. –  Pete Carter Nov 9 '12 at 7:51
    
As a side question, any idea why SQL doesn't reorganise indexes in the background during low demand periods (i.e. to avoid requiring scheduled tasks or manually requesting this action)? –  JohnLBevan Apr 6 '13 at 17:40
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