Issue
SQL Queries taking longer than normal to complete.
Product
Struxureware Power Monitoring
Power Monitoring Expert
ION Enterprise
ION EEM
Environment
SQL Server 2005, SQL Server 2008 R2, SQL Server 2012, SQL Server 2014, SQL Server 2016
Cause
when a database is frequently updated via INSERT, UPDATE, or DELETE statements, over time these modifications can cause the information in the index to become scattered in the database (fragmented). Fragmentation exists when indexes have pages in which the logical ordering, based on the key value, does not match the physical ordering inside the data file.
If database indexes are fragmented, the SQL Server query optimizer may choose a non-optimal execution plan when using an index to resolve a query. Heavily fragmented indexes can degrade query performance and cause your application to respond slowly.
Resolution
*Warning: Irreparable database damage can occur. This procedure should only be performed by users familiar with SQL Server Management Studio. Databases should be backed up prior to performing this procedure.*
The following is a simple query that will list every index on every table in your database, ordered by the percentage of index fragmentation.
SELECT dbschemas.[name] as 'Schema',
dbtables.[name] as 'Table',
dbindexes.[name] as 'Index',
indexstats.avg_fragmentation_in_percent,
indexstats.page_count
FROM sys.dm_db_index_physical_stats (DB_ID(), NULL, NULL, NULL, NULL) AS indexstats
INNER JOIN sys.tables dbtables on dbtables.[object_id] = indexstats.[object_id]
INNER JOIN sys.schemas dbschemas on dbtables.[schema_id] = dbschemas.[schema_id]
INNER JOIN sys.indexes AS dbindexes ON dbindexes.[object_id] = indexstats.[object_id]
AND indexstats.index_id = dbindexes.index_id
WHERE indexstats.database_id = DB_ID()
ORDER BY indexstats.avg_fragmentation_in_percent desc
This query can be modified to focus on specific tables by appending the table name to the 'WHERE' clause:
WHERE indexstats.database_id = DB_ID() and dbtables.[name] like '%%'
Depending on the index fragmentation percentage, fragmentation can be reduced by reorganizing or rebuilding. For heavily fragmented indexes (greater than 30%) a rebuilding process is needed, else index reorganizing is sufficient. The following table summarizes when to use each one:
| Reference Values (in %) |
Action |
SQL statement |
| avg_fragmentation_in_percent > 5 AND < 30 |
Reorganize Index |
ALTER INDEX REORGANIZE |
| avg_fragmentation_in_percent > 30 |
Rebuild Index |
ALTER INDEX REBUILD |
Indexes can be rebuilt/reorganize in SQL Server Management Studio using the following steps;
1- In Object Explorer locate the table that holds the intended index and expand it.
2- Expand Indexes.
3- Right-click on the index and click on Rebuild or Reorganize.
