Results 1 to 4 of 4
  1. #1
    Join Date
    Nov 2002
    Posts
    71

    Unanswered: DBREINDEX/INDEXDEFRAG a few questions

    I pretty much understand the differences between DBCC DBREINDEX and DBCC INDEXDEFRAG. However, I need the forums help to understand a few specific issues relating to clustered/non-clustered indexes and the advantages/disadvantages of running the DBCC DBREINDEX/INDEXDEFRAG against the table or against each specific index...

    "If a table has a clustered index, it's only necessary to re-index the clustered index because any non-clustered indexes on that table will be automatically re-indexed as well."

    I think the above statement is true for DBCC DBREINDEX but is the following statement true for DBCC INDEXDEFRAG:-

    "If a table has a clustered index, it's only necessary to Index Defrag the clustered index because any non-clustered indexes on that table will be automatically defragged as well."

    Following on from the above, is there any advantage with an index maintenance strategy to individually running DBCC DBREINDEX against each specific index as opposed to running it against the table and letting SQL sort out the underlying indexes? Does the same apply to DBCC INDEXDEFRAG?

    Regards,

    Clive

  2. #2
    Join Date
    Jan 2003
    Location
    Massachusetts
    Posts
    5,800
    Provided Answers: 11
    Quote Originally Posted by crichardson
    "If a table has a clustered index, it's only necessary to Index Defrag the clustered index because any non-clustered indexes on that table will be automatically defragged as well."
    I just did a test of this, and the answer is no, the non-clustered indexes will not be defragged as well. This works out as a slight benefit to indexdefrag, as it saves you the time of rebuilding the non-clustered indexes at the same time as the clustered index.

    If you have all non-clustered indexes, then there would be a slight benefit in space-savings by running dbcc dbreindex on each index separately. If there is a clustered index, then running dbreindex on the clustered index rebuilds all of the indexes. This would be a waste of any time spent on each index being rebuilt individually. Does this help?

  3. #3
    Join Date
    Nov 2002
    Posts
    71
    Yes, that helps. Thank you. In fact, I just put together a test myself and found the same result. Not unsurprisingly, the DBREINDEX produced 100% scan density on the clustered index and the same or close to it on the non-clustered indexes. However, I was surprised that INDEXDEFRAG on the clustered index actually lowered scan-density by a few percent!

    Regards,

    Clive

  4. #4
    Join Date
    Jan 2003
    Location
    Massachusetts
    Posts
    5,800
    Provided Answers: 11
    What was the scan density when you started?

    The main problem with indexdefrag is that it is only moving pages from and to page locations that are already allocated to the index being defragmented. It may coalesce some unused space among these allocations, but it does not touch anything that is already allocated to another index page or data page of the table. Since it has to work around all of these prior allocations, you almost never get a nice 100% result from indexdefrag, unless all you had to start with was the clustered index.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •