Software Development

Optimising clustered indexes in SQL Server 2008

I’ve just found a script on another blog to go through all the clustered indexes in a SQL Server database and rebuild them in order to reduce fragmentation and improve the disk IO needed to get the data. The original script didn’t take into account tables in different schemas so I updated it. I also added some metrics to it so I could get a sense of how long the operation takes on each table.

The script can take quite a while to run. On my database it took over 6 minutes just to initially run the query to work out what needed rebuilding, and each index can take several seconds (or possibly more if you have a lot of data) on its own.

The new script is here:

SET NOCOUNT ON
DECLARE @Schema SYSNAME;
DECLARE @Table SYSNAME;
DECLARE @Index SYSNAME;
DECLARE @Rebuild NVARCHAR(4000)
DECLARE @StartTime DATETIME = GETUTCDATE();

PRINT (CONVERT(NVARCHAR(100), GETUTCDATE(), 113) + ' : Rebuild all indexes with over 10% fragmentation.')

DECLARE DB CURSOR FOR 
SELECT SS.name [schema], SO.Name [table], SI.Name [index]
FROM SYS.dm_db_index_physical_stats (DB_ID(), NULL, NULL , NULL, 'LIMITED')
INNER JOIN SYS.objects SO ON SO.object_id=SYS.dm_db_index_physical_stats.object_id
INNER JOIN SYS.schemas SS ON SO.schema_id=SS.schema_id
INNER JOIN SYS.indexes SI ON SI.index_id=SYS.dm_db_index_physical_stats.index_id AND 
SI.object_id=sys.dm_db_index_physical_stats.object_id 
-- Begin select only clustered indexes Index_id = 1
AND SI.Index_id = 1
-- End select only clustered indexes Index_id = 1
WHERE avg_fragmentation_in_percent > 10.0 AND SYS.dm_db_index_physical_stats.index_id > 0
ORDER BY SO.Name 

OPEN DB
FETCH NEXT FROM DB INTO @Schema, @Table, @Index
WHILE @@FETCH_STATUS = 0
BEGIN
  SET @Rebuild = 'ALTER INDEX ' + @Index + ' ON ' + @Schema + '.' + @Table + ' REBUILD'

  PRINT (CONVERT(NVARCHAR(100), GETUTCDATE(), 113) + ' : ' + @Rebuild)

  -- Comment out the following line to see what tables would be affected without rebuilding the indexes
  EXEC SP_EXECUTESQL @Rebuild

  FETCH NEXT FROM DB INTO @Schema, @Table, @Index
END
CLOSE DB
DEALLOCATE DB

DECLARE @Duration DATETIME = GETUTCDATE() - @StartTime;
PRINT (CONVERT(NVARCHAR(100), GETUTCDATE(), 113) + ' : Finished. Duration = '+CONVERT(NVARCHAR(100), @Duration, 114))

SET NOCOUNT OFF

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