Question

What is Index Maintenance and how do I do it? How frequently do I have to do it?
What are the benefits?
This is related to a transaction table which is subject to frequently modifications; all DML operations will be run on that table.

Answer 1

I second everything that Jonathan said – except for the frequency of index maintenance.

Well, if you happen to have a poorly designed index (such as a clustered index on a GUID key), you might actually need to do it at least every night – or even during the day, too.

As a general rule of thumb: if your index fragmentation is below 5%, all is fine. If you have fragmentation between 5% and approx. 30%, you should do an index reorganization:

ALTER INDEX (your index name) ON (your table name) REORGANIZE

If your index has index fragmentation of more than 30%, you need to rebuild it completely:

ALTER INDEX (your index name) ON (your table name) REBUILD

Rebuilding an index can be disruptive – try to do it at off-hours, e.g. during the night.

In order to determine index fragmentation, you can use this DMV query:

SELECT 
    t.NAME 'Table name',
    i.NAME 'Index name',
    ips.index_type_desc,
    ips.alloc_unit_type_desc,
    ips.index_depth,
    ips.index_level,
    ips.avg_fragmentation_in_percent,
    ips.fragment_count,
    ips.avg_fragment_size_in_pages,
    ips.page_count,
    ips.avg_page_space_used_in_percent,
    ips.record_count,
    ips.ghost_record_count,
    ips.Version_ghost_record_count,
    ips.min_record_size_in_bytes,
    ips.max_record_size_in_bytes,
    ips.avg_record_size_in_bytes,
    ips.forwarded_record_count
FROM 
    sys.dm_db_index_physical_stats(DB_ID(), NULL, NULL, NULL, 'DETAILED') ips
INNER JOIN  
    sys.tables t ON ips.OBJECT_ID = t.Object_ID
INNER JOIN  
    sys.indexes i ON ips.index_id = i.index_id AND ips.OBJECT_ID = i.object_id
WHERE
    AVG_FRAGMENTATION_IN_PERCENT > 0.0
ORDER BY
    AVG_FRAGMENTATION_IN_PERCENT, fragment_count

Michelle Ufford has a great automatic index defrag script – highly recommended! Or then you should look into setting up SQL Server maintenance plans which can run e.g. every night and clean up your indices.

Marc