Sql-server – Rebuild or Reorganize indexes based on fragmentation

Defragmentation strategies help improve scan speed to/from disk.

The wide variety of opinions is because an environment's ideal defragmentation strategy should depends on many different factors. There are also multiple potential layers of fragmentation in play.

Saying that your databases are stored on a SAN isn't enough information. For example:

• Are database files stored on separate physical RAID groups or the same RAID group? What other processes are active on that same device? Are your backup files ending up there, too? You may have to ask your SAN admin for this information, because it's not always transparent.

• What are the access patterns for the databases? OLTP is generally random access, but sometimes an application is table-scan-happy and you can't change its behaviour (ISV app). Are the applications read-mostly, write-mostly, or somewhere in between?

• Are there performance SLAs in play during a recovery/failover period?

Brent's post assumes there is one giant pool of storage and everything shares it. This means the physical disks are rarely idle, and hence most access is random. If that is your situation, then the advice applies, and I agree with it for the most part. While this type of strategy is much easier to manage, it isn't necessarily (a) what you have in your environment, or (b) what is the best solution for your environment.

If index maintenance is burdensome, consider doing it less aggressively, and/or amortize the cost over the week (i.e., run light maintenance once/day, instead of heavy maintenance once/week).

You can also turn on the SortInTempdb option to potentially reduce the amount of logging that takes place in the user databases.

Reorganizing an index does not update the statistics on an index. Reorganize will ensure that the B+ tree is balanced, since SQL Server doesn't auto balance trees during operation. Rebuilding an index will recreate the index, allowing the engine to update the statistics on an index.

You should consider periodically reorganizing and updating statistics on lightly fragmented queries granted you can spare the maintenance time. You can also try to update statistics periodically using samples, if your data is homogenous enough.

You can read more about Update Statistics here

http://msdn.microsoft.com/en-us/library/ms187348.aspx

Kimberly Tripp blogs heavily on index maintenance on http://sqlskills.com

Check out her links for Statistics and Indexes.