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Monday, September 12, 2011

Defragmenting SQL Server Indexes


"As of this moment, we are at war!" - Admiral William Adama, Reimagined Battlestar Galactica Mini-Series

Kind of a basic tip but it's something that I needed to store here for my reference anyway.

How do you identify your fragmented indexes and then fix them?

The following query will identify fragmented indexes where the fragmentation percent is greater than 10% and there's more than 100 pages in the index. Conventional wisdom is to pretty much exclude indexes from

SELECT ps.database_id, ps.OBJECT_ID,
ps.index_id, b.name,
ps.avg_fragmentation_in_percent, b.name, ps.page_count
FROM sys.dm_db_index_physical_stats (DB_ID(), NULL, NULL, NULL, NULL) AS ps
INNER JOIN sys.indexes AS b ON ps.OBJECT_ID = b.OBJECT_ID
AND ps.index_id = b.index_id
WHERE ps.database_id = DB_ID()
and ps.avg_fragmentation_in_percent > 10
and ps.page_count > 100
ORDER BY b.name
I have the fortunate ability to be able to rebuild my indexes during monthly maintenance windows. The below script would not be wise to run during system uptime because it's going to cause locks while indexes are being rebuilt.

However, since I have the luxury of a regular maintenance window, I do my rebuilds then.

SET NOCOUNT ON;
DECLARE @objectid int;
DECLARE @indexid int;
DECLARE @partitioncount bigint;
DECLARE @schemaname sysname;
DECLARE @objectname sysname;
DECLARE @indexname sysname;
DECLARE @partitionnum bigint;
DECLARE @partitions bigint;
DECLARE @frag float;
DECLARE @command varchar(8000);
-- ensure the temporary table does not exist
IF EXISTS (SELECT name FROM sys.objects WHERE name = 'work_to_do')
    DROP TABLE work_to_do;
-- conditionally select from the function, converting object and 
-- index IDs to names.
SELECT
    object_id AS objectid,
    index_id AS indexid,
    partition_number AS partitionnum,
    avg_fragmentation_in_percent AS frag
INTO work_to_do
FROM sys.dm_db_index_physical_stats (DB_ID(), NULL, NULL , NULL, 'LIMITED')
WHERE avg_fragmentation_in_percent > 10.0 AND index_id > 0;
-- Declare the cursor for the list of partitions to be processed.
DECLARE partitions CURSOR FOR SELECT * FROM work_to_do;

-- Open the cursor.
OPEN partitions;

-- Loop through the partitions.
FETCH NEXT
   FROM partitions
   INTO @objectid, @indexid, @partitionnum, @frag;

WHILE @@FETCH_STATUS = 0
    BEGIN;
        SELECT @objectname = o.name, @schemaname = s.name
        FROM sys.objects AS o
        JOIN sys.schemas as s ON s.schema_id = o.schema_id
        WHERE o.object_id = @objectid;

        SELECT @indexname = name 
        FROM sys.indexes
        WHERE  object_id = @objectid AND index_id = @indexid;

        SELECT @partitioncount = count (*) 
        FROM sys.partitions
        WHERE object_id = @objectid AND index_id = @indexid;

-- 30 is an arbitrary decision point at which to switch between 
-- reorganizing and rebuilding

IF @frag < 30.0
    BEGIN;
    SELECT @command = 'ALTER INDEX ' + @indexname + ' ON ' + @schemaname +
                  '.' + @objectname + ' REORGANIZE';
    IF @partitioncount > 1
        SELECT @command = @command + ' PARTITION=' + 
                      CONVERT (CHAR, @partitionnum);
    EXEC (@command);
    END;

IF @frag >= 30.0
    BEGIN;
    SELECT @command = 'ALTER INDEX ' + @indexname +' ON ' + @schemaname +
                  '.' + @objectname + ' REBUILD';
    IF @partitioncount > 1
        SELECT @command = @command + ' PARTITION=' + 
                     CONVERT (CHAR, @partitionnum);
    EXEC (@command);
    END;
PRINT 'Executed ' + @command;

FETCH NEXT FROM partitions INTO @objectid, @indexid, @partitionnum, @frag;
END;
-- Close and deallocate the cursor.
CLOSE partitions;
DEALLOCATE partitions;

-- drop the temporary table
IF EXISTS (SELECT name FROM sys.objects WHERE name = 'work_to_do')
    DROP TABLE work_to_do;
GO

The actual "how and why" behind why an index gets fragmented is something for another time.

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