Page Splitting & Rollbacks
Filed under: Internals, Performance & Tuning, Syndication
So while I was at the grocery store last night, my mind wandered to SQL Server. This time, I was pondering what happens to a page split if the transaction is rolled back. I didn’t know the answer, but my guess was that the page split remained, since it would be less expensive for SQL Server to leave the data where it was sitting. Also, in theory, if the page split occurred once, it could need to occur again, so why undergo that same expense twice?
I decided to run a simple test to see what would happen. First, I created a really wide table and tossed 4 rows into it:
CREATE TABLE myTable ( id INT PRIMARY KEY , wideColumn CHAR(2000) ); INSERT INTO myTable SELECT 1, REPLICATE('A', 2000) UNION All SELECT 2, REPLICATE('B', 2000) UNION All SELECT 4, REPLICATE('C', 2000) UNION All SELECT 5, REPLICATE('D', 2000);
I left an open spot for id=3, so I can force a page split. Next, I looked at the page data using the script I posted in February.
Here’s what I saw:
Pay attention to the 2 items boxed in red. m_slotCnt tells us how many records are on the page, and m_nextPage tells us the address of the next page. As you may have guessed, a m_nextPage value of 0:0 means you’re looking at the last page allocated to the object.
Now let’s insert a record, roll it back, and see what happens:
BEGIN TRANSACTION; INSERT INTO myTable SELECT 3, REPLICATE('E', 2000); ROLLBACK TRANSACTION;
I ran my DBCC Page command again and here’s what I saw:
As you can see, m_slotCnt is now 2, and m_nextPage is no longer 0:0 (although your actual page number will probably be different than mine). If I pull up the new page, I find my 2 relocated records, id’s 4 and 5.
So what’s this all mean? In short, page splits are NOT reversed when a transaction is rolled back. Why should you care? Well, you probably wouldn’t care much, unless you roll back a lot of transactions. But this is also a good thing to keep in mind if you have to abort a really large insert or update; if you don’t plan to re-execute the aborted script, you may want to defrag your indexes to fix the splits.
Source: http://sqlfool.com/2009/04/page-splitting-rollbacks/
Automated Index Defrag Script
Filed under: Performance & Tuning, SQL 2008, SQL Tips, T-SQL Scripts
Note: This script has been updated. You can find the latest version here: http://sqlfool.com/2009/06/index-defrag-script-v30/
So after much delay, here’s the latest and greatest version of my index defrag script.
A summary of the changes:
- Added support for centralized execution. Option to defrag indexes for a single database from another database, or for all non-system databases.
- Consolidated Enterprise and Standard versions of the script with new @editionCheck variable.
- Added parametrization for MaxDop restrictions during rebuilds; validates that the value does not exceed the actual number of processors on the server.
- Changed minimum fragmentation default value from 10 to 5 to match BOL recommendations.
- Limited defrags to objects with more than 8 pages.
- Added a debug option to give a little more insight into what’s happening and to assist with troubleshooting.
- Updated logic for handling partitions and LOBs.
And a couple of notes and explanations:
Don’t you know you can just pass NULL to sys.dm_db_index_physical_stats for the DatabaseID?
Yes, I realize you can do this. But I don’t want to defrag the system databases, i.e. tempdb, so I opted to handle it this way instead.
How long will this take?
It depends. I don’t necessarily recommend running it without specifying a database; at least, not unmonitored. You *can* do that, but it could take a while. For example, to run sys.dm_db_index_physical_stats for all databases and tables, totaling 2TB, took me 4.5 hours; that doesn’t even count the actual defrags.
Where should I put this?
It’s up to you. If you have a database for items like centralized maintenance or scratch tables, that may be a good place for it. If you prefer, you could also put this in each individual database and call it locally, too. I would not put this in the master or msdb databases.
This is pretty close to a complete rewrite, so please let me know if you encounter any bugs. And now… the code!
IF Not Exists(SELECT [OBJECT_ID] FROM sys.tables WHERE name = N'dba_indexDefragLog') BEGIN -- Drop Table dbo.dba_indexDefragLog CREATE TABLE dbo.dba_indexDefragLog ( indexDefrag_id INT IDENTITY(1,1) Not Null , databaseID INT Not Null , databaseName NVARCHAR(128) Not Null , objectID INT Not Null , objectName NVARCHAR(128) Not Null , indexID INT Not Null , indexName NVARCHAR(128) Not Null , partitionNumber SMALLINT Not Null , fragmentation FLOAT Not Null , page_count INT Not Null , dateTimeStart DATETIME Not Null , durationSeconds INT Not Null CONSTRAINT PK_indexDefragLog PRIMARY KEY CLUSTERED (indexDefrag_id) ) PRINT 'dba_indexDefragLog Table Created'; END IF OBJECTPROPERTY(OBJECT_ID('dbo.dba_indexDefrag_sp'), N'IsProcedure') = 1 BEGIN DROP PROCEDURE dbo.dba_indexDefrag_sp; PRINT 'Procedure dba_indexDefrag_sp dropped'; END; Go CREATE PROCEDURE dbo.dba_indexDefrag_sp /* Declare Parameters */ @minFragmentation FLOAT = 5.0 /* in percent, will not defrag if fragmentation less than specified */ , @rebuildThreshold FLOAT = 30.0 /* in percent, greater than @rebuildThreshold will result in rebuild instead of reorg */ , @executeSQL BIT = 1 /* 1 = execute; 0 = print command only */ , @DATABASE VARCHAR(128) = Null /* Option to specify a database name; null will return all */ , @tableName VARCHAR(4000) = Null -- databaseName.schema.tableName /* Option to specify a table name; null will return all */ , @onlineRebuild BIT = 1 /* 1 = online rebuild; 0 = offline rebuild; only in Enterprise */ , @maxDopRestriction TINYINT = Null /* Option to restrict the number of processors for the operation; only in Enterprise */ , @printCommands BIT = 0 /* 1 = print commands; 0 = do not print commands */ , @printFragmentation BIT = 0 /* 1 = print fragmentation prior to defrag; 0 = do not print */ , @defragDelay CHAR(8) = '00:00:05' /* time to wait between defrag commands */ , @debugMode BIT = 0 /* display some useful comments to help determine if/where issues occur */ AS /********************************************************************************* Name: dba_indexDefrag_sp Author: Michelle Ufford, http://sqlfool.com Purpose: Defrags all indexes for one or more databases Notes: CAUTION: TRANSACTION LOG SIZE MUST BE MONITORED CLOSELY WHEN DEFRAGMENTING. @minFragmentation defaulted to 10%, will not defrag if fragmentation is less than that @rebuildThreshold defaulted to 30% as recommended by Microsoft in BOL; greater than 30% will result in rebuild instead @executeSQL 1 = execute the SQL generated by this proc; 0 = print command only @database Optional, specify specific database name to defrag; If not specified, all non-system databases will be defragged. @tableName Specify if you only want to defrag indexes for a specific table, format = databaseName.schema.tableName; if not specified, all tables will be defragged. @onlineRebuild 1 = online rebuild; 0 = offline rebuild @maxDopRestriction Option to specify a processor limit for index rebuilds @printCommands 1 = print commands to screen; 0 = do not print commands @printFragmentation 1 = print fragmentation to screen; 0 = do not print fragmentation @defragDelay time to wait between defrag commands; gives the server a little time to catch up @debugMode 1 = display debug comments; helps with troubleshooting 0 = do not display debug comments Called by: SQL Agent Job or DBA Date Initials Description ---------------------------------------------------------------------------- 2008-10-27 MFU Initial Release for public consumption 2008-11-17 MFU Added page-count to log table , added @printFragmentation option 2009-03-17 MFU Provided support for centralized execution, , consolidated Enterprise & Standard versions , added @debugMode, @maxDopRestriction , modified LOB and partition logic ********************************************************************************* Exec dbo.dba_indexDefrag_sp @executeSQL = 0 , @minFragmentation = 80 , @printCommands = 1 , @debugMode = 1 , @printFragmentation = 1 , @database = 'AdventureWorks' , @tableName = 'AdventureWorks.Sales.SalesOrderDetail'; *********************************************************************************/ SET NOCOUNT ON; SET XACT_Abort ON; SET Quoted_Identifier ON; BEGIN IF @debugMode = 1 RAISERROR('Dusting off the spiderwebs and starting up...', 0, 42) WITH NoWait; /* Declare our variables */ DECLARE @objectID INT , @databaseID INT , @databaseName NVARCHAR(128) , @indexID INT , @partitionCount BIGINT , @schemaName NVARCHAR(128) , @objectName NVARCHAR(128) , @indexName NVARCHAR(128) , @partitionNumber SMALLINT , @partitions SMALLINT , @fragmentation FLOAT , @pageCount INT , @sqlCommand NVARCHAR(4000) , @rebuildCommand NVARCHAR(200) , @dateTimeStart DATETIME , @dateTimeEnd DATETIME , @containsLOB BIT , @editionCheck BIT , @debugMessage VARCHAR(128) , @updateSQL NVARCHAR(4000) , @partitionSQL NVARCHAR(4000) , @partitionSQL_Param NVARCHAR(1000) , @LOB_SQL NVARCHAR(4000) , @LOB_SQL_Param NVARCHAR(1000); /* Create our temporary tables */ CREATE TABLE #indexDefragList ( databaseID INT , databaseName NVARCHAR(128) , objectID INT , indexID INT , partitionNumber SMALLINT , fragmentation FLOAT , page_count INT , defragStatus BIT , schemaName NVARCHAR(128) Null , objectName NVARCHAR(128) Null , indexName NVARCHAR(128) Null ); CREATE TABLE #databaseList ( databaseID INT , databaseName VARCHAR(128) ); CREATE TABLE #processor ( [INDEX] INT , Name VARCHAR(128) , Internal_Value INT , Character_Value INT ); IF @debugMode = 1 RAISERROR('Beginning validation...', 0, 42) WITH NoWait; /* Just a little validation... */ IF @minFragmentation Not Between 0.00 And 100.0 SET @minFragmentation = 5.0; IF @rebuildThreshold Not Between 0.00 And 100.0 SET @rebuildThreshold = 30.0; IF @defragDelay Not Like '00:[0-5][0-9]:[0-5][0-9]' SET @defragDelay = '00:00:05'; /* Make sure we're not exceeding the number of processors we have available */ INSERT INTO #processor EXECUTE XP_MSVER 'ProcessorCount'; IF @maxDopRestriction IS Not Null And @maxDopRestriction > (SELECT Internal_Value FROM #processor) SELECT @maxDopRestriction = Internal_Value FROM #processor; /* Check our server version; 1804890536 = Enterprise, 610778273 = Enterprise Evaluation, -2117995310 = Developer */ IF (SELECT SERVERPROPERTY('EditionID')) In (1804890536, 610778273, -2117995310) SET @editionCheck = 1 -- supports online rebuilds ELSE SET @editionCheck = 0; -- does not support online rebuilds IF @debugMode = 1 RAISERROR('Grabbing a list of our databases...', 0, 42) WITH NoWait; /* Retrieve the list of databases to investigate */ INSERT INTO #databaseList SELECT database_id , name FROM sys.databases WHERE name = IsNull(@DATABASE, name) And database_id > 4 -- exclude system databases And [STATE] = 0; -- state must be ONLINE IF @debugMode = 1 RAISERROR('Looping through our list of databases and checking for fragmentation...', 0, 42) WITH NoWait; /* Loop through our list of databases */ WHILE (SELECT COUNT(*) FROM #databaseList) > 0 BEGIN SELECT TOP 1 @databaseID = databaseID FROM #databaseList; SELECT @debugMessage = ' working on ' + DB_NAME(@databaseID) + '...'; IF @debugMode = 1 RAISERROR(@debugMessage, 0, 42) WITH NoWait; /* Determine which indexes to defrag using our user-defined parameters */ INSERT INTO #indexDefragList SELECT database_id AS databaseID , QUOTENAME(DB_NAME(database_id)) AS 'databaseName' , [OBJECT_ID] AS objectID , index_id AS indexID , partition_number AS partitionNumber , avg_fragmentation_in_percent AS fragmentation , page_count , 0 AS 'defragStatus' /* 0 = unprocessed, 1 = processed */ , Null AS 'schemaName' , Null AS 'objectName' , Null AS 'indexName' FROM sys.dm_db_index_physical_stats (@databaseID, OBJECT_ID(@tableName), Null , Null, N'Limited') WHERE avg_fragmentation_in_percent >= @minFragmentation And index_id > 0 -- ignore heaps And page_count > 8 -- ignore objects with less than 1 extent OPTION (MaxDop 1); DELETE FROM #databaseList WHERE databaseID = @databaseID; END CREATE CLUSTERED INDEX CIX_temp_indexDefragList ON #indexDefragList(databaseID, objectID, indexID, partitionNumber); SELECT @debugMessage = 'Looping through our list... there''s ' + CAST(COUNT(*) AS VARCHAR(10)) + ' indexes to defrag!' FROM #indexDefragList; IF @debugMode = 1 RAISERROR(@debugMessage, 0, 42) WITH NoWait; /* Begin our loop for defragging */ WHILE (SELECT COUNT(*) FROM #indexDefragList WHERE defragStatus = 0) > 0 BEGIN IF @debugMode = 1 RAISERROR(' Picking an index to beat into shape...', 0, 42) WITH NoWait; /* Grab the most fragmented index first to defrag */ SELECT TOP 1 @objectID = objectID , @indexID = indexID , @databaseID = databaseID , @databaseName = databaseName , @fragmentation = fragmentation , @partitionNumber = partitionNumber , @pageCount = page_count FROM #indexDefragList WHERE defragStatus = 0 ORDER BY fragmentation DESC; IF @debugMode = 1 RAISERROR(' Looking up the specifics for our index...', 0, 42) WITH NoWait; /* Look up index information */ SELECT @updateSQL = N'Update idl Set schemaName = QuoteName(s.name) , objectName = QuoteName(o.name) , indexName = QuoteName(i.name) From #indexDefragList As idl Inner Join ' + @databaseName + '.sys.objects As o On idl.objectID = o.object_id Inner Join ' + @databaseName + '.sys.indexes As i On o.object_id = i.object_id Inner Join ' + @databaseName + '.sys.schemas As s On o.schema_id = s.schema_id Where o.object_id = ' + CAST(@objectID AS VARCHAR(10)) + ' And i.index_id = ' + CAST(@indexID AS VARCHAR(10)) + ' And i.type > 0 And idl.databaseID = ' + CAST(@databaseID AS VARCHAR(10)); EXECUTE SP_EXECUTESQL @updateSQL; /* Grab our object names */ SELECT @objectName = objectName , @schemaName = schemaName , @indexName = indexName FROM #indexDefragList WHERE objectID = @objectID And indexID = @indexID And databaseID = @databaseID; IF @debugMode = 1 RAISERROR(' Grabbing the partition count...', 0, 42) WITH NoWait; /* Determine if the index is partitioned */ SELECT @partitionSQL = 'Select @partitionCount_OUT = Count(*) From ' + @databaseName + '.sys.partitions Where object_id = ' + CAST(@objectID AS VARCHAR(10)) + ' And index_id = ' + CAST(@indexID AS VARCHAR(10)) + ';' , @partitionSQL_Param = '@partitionCount_OUT int OutPut'; EXECUTE SP_EXECUTESQL @partitionSQL, @partitionSQL_Param, @partitionCount_OUT = @partitionCount OUTPUT; IF @debugMode = 1 RAISERROR(' Seeing if there''s any LOBs to be handled...', 0, 42) WITH NoWait; /* Determine if the table contains LOBs */ SELECT @LOB_SQL = ' Select Top 1 @containsLOB_OUT = column_id From ' + @databaseName + '.sys.columns With (NoLock) Where [object_id] = ' + CAST(@objectID AS VARCHAR(10)) + ' And (system_type_id In (34, 35, 99) Or max_length = -1);' /* system_type_id --> 34 = image, 35 = text, 99 = ntext max_length = -1 --> varbinary(max), varchar(max), nvarchar(max), xml */ , @LOB_SQL_Param = '@containsLOB_OUT int OutPut'; EXECUTE SP_EXECUTESQL @LOB_SQL, @LOB_SQL_Param, @containsLOB_OUT = @containsLOB OUTPUT; IF @debugMode = 1 RAISERROR(' Building our SQL statements...', 0, 42) WITH NoWait; /* If there's not a lot of fragmentation, or if we have a LOB, we should reorganize */ IF @fragmentation < @rebuildThreshold Or @containsLOB = 1 Or @partitionCount > 1 BEGIN SET @sqlCommand = N'Alter Index ' + @indexName + N' On ' + @databaseName + N'.' + @schemaName + N'.' + @objectName + N' ReOrganize'; /* If our index is partitioned, we should always reorganize */ IF @partitionCount > 1 SET @sqlCommand = @sqlCommand + N' Partition = ' + CAST(@partitionNumber AS NVARCHAR(10)); END; /* If the index is heavily fragmented and doesn't contain any partitions or LOB's, rebuild it */ IF @fragmentation >= @rebuildThreshold And IsNull(@containsLOB, 0) != 1 And @partitionCount <= 1 BEGIN /* Set online rebuild options; requires Enterprise Edition */ IF @onlineRebuild = 1 And @editionCheck = 1 SET @rebuildCommand = N' Rebuild With (Online = On'; ELSE SET @rebuildCommand = N' Rebuild With (Online = Off'; /* Set processor restriction options; requires Enterprise Edition */ IF @maxDopRestriction IS Not Null And @editionCheck = 1 SET @rebuildCommand = @rebuildCommand + N', MaxDop = ' + CAST(@maxDopRestriction AS VARCHAR(2)) + N')'; ELSE SET @rebuildCommand = @rebuildCommand + N')'; SET @sqlCommand = N'Alter Index ' + @indexName + N' On ' + @databaseName + N'.' + @schemaName + N'.' + @objectName + @rebuildCommand; END; /* Are we executing the SQL? If so, do it */ IF @executeSQL = 1 BEGIN IF @debugMode = 1 RAISERROR(' Executing SQL statements...', 0, 42) WITH NoWait; /* Grab the time for logging purposes */ SET @dateTimeStart = GETDATE(); EXECUTE SP_EXECUTESQL @sqlCommand; SET @dateTimeEnd = GETDATE(); /* Log our actions */ INSERT INTO dbo.dba_indexDefragLog ( databaseID , databaseName , objectID , objectName , indexID , indexName , partitionNumber , fragmentation , page_count , dateTimeStart , durationSeconds ) SELECT @databaseID , @databaseName , @objectID , @objectName , @indexID , @indexName , @partitionNumber , @fragmentation , @pageCount , @dateTimeStart , DATEDIFF(SECOND, @dateTimeStart, @dateTimeEnd); /* Just a little breather for the server */ WAITFOR Delay @defragDelay; /* Print if specified to do so */ IF @printCommands = 1 PRINT N'Executed: ' + @sqlCommand; END ELSE /* Looks like we're not executing, just printing the commands */ BEGIN IF @debugMode = 1 RAISERROR(' Printing SQL statements...', 0, 42) WITH NoWait; IF @printCommands = 1 PRINT IsNull(@sqlCommand, 'error!'); END IF @debugMode = 1 RAISERROR(' Updating our index defrag status...', 0, 42) WITH NoWait; /* Update our index defrag list so we know we've finished with that index */ UPDATE #indexDefragList SET defragStatus = 1 WHERE databaseID = @databaseID And objectID = @objectID And indexID = @indexID And partitionNumber = @partitionNumber; END /* Do we want to output our fragmentation results? */ IF @printFragmentation = 1 BEGIN IF @debugMode = 1 RAISERROR(' Displaying fragmentation results...', 0, 42) WITH NoWait; SELECT databaseID , databaseName , objectID , objectName , indexID , indexName , fragmentation , page_count FROM #indexDefragList; END; /* When everything is said and done, make sure to get rid of our temp table */ DROP TABLE #indexDefragList; DROP TABLE #databaseList; DROP TABLE #processor; IF @debugMode = 1 RAISERROR('DONE! Thank you for taking care of your indexes! :)', 0, 42) WITH NoWait; SET NOCOUNT OFF; RETURN 0 END Go
Thanks to my beta testers, @scoinva, @davidmtate, @jdanton, and @SuperCoolMoss!
Special thanks to SCM for keeping on me to finish this.
Happy Defragging!
Michelle
Source: http://sqlfool.com/2009/03/automated-index-defrag-script/
Index Fragmentation PotW Webcast – Now Available
Filed under: Miscellaneous, Performance & Tuning, Presentations, Syndication
The Pain-of-the-Week webcast I did last Thursday is now online! You can find it here: http://www.quest.com/common/registration.aspx?requestdefid=23092.
The topic of the webcast is index fragmentation: what is it, how to identify it, and how to fix it. You can find the materials for this webcast in my previous blog post. I’ll also be updating SQLServerPedia with the materials, hopefully tonight.
Thanks to the great feedback I’ve received, I’ll be doing another Pain-of-the-Week webcast, this time with Brent Ozar. More info to follow as we hammer out the specifics.
In a related note, I have updated my index defrag script. I’m just waiting for my awesome beta testers to let me know everything works for them. It works just fine on my servers (lol) but I don’t have access to any Standard editions, etc., so I just want to be extra sure before I unleash it on the SQL world. I should have it online tomorrow.
Source: http://sqlfool.com/2009/03/index-fragmentation-potw-webcast/
Fragmentation on Replicated Tables
Filed under: Miscellaneous, Performance & Tuning, SQL Tips, Syndication
I’ve heard more than one person state that they do not need to defrag their tables, because they’re replicated and the source tables are defragged regularly. Let’s set this myth straight.
Repeat after me: defragmentation processes are not replicated.
Good. Thank you.
For those non-believers, let me prove it:
SET NOCOUNT ON; /* Create publication db */ CREATE DATABASE sandbox_publisher; GO /* Create subscription db */ CREATE DATABASE sandbox_subscriber; GO /* Navigate to publisher db */ USE sandbox_publisher; GO /* Create a table to play with */ CREATE TABLE myTable ( myID UNIQUEIDENTIFIER , myColumn VARCHAR(2000) CONSTRAINT PK_myTable PRIMARY KEY CLUSTERED (myID) ); /* NOTE: please never, ever use this type of clustered index in real-life. The whole purpose is to generate substantial fragmentation for illustrative purposes. */ /* Populate it with some non-sequential records to generate fragmentation */ DECLARE @iteration INT; SET @iteration = 0; WHILE @iteration < 1000 BEGIN INSERT INTO myTable SELECT NewID(), REPLICATE('A', 2000); SET @iteration = @iteration + 1; END /* Now let's verify that we really did make a mess of our clustered index */ SELECT avg_fragmentation_in_percent , page_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID(N'MyTable'), NULL , NULL, N'Limited') OPTION (MaxDop 1); /* Results: avg_fragmentation_in_percent page_count ---------------------------- -------------------- 99.0157480314961 508 99% fragmented, not too shabby! Your results may vary slightly, based on the guid values generated by NewID(). */ /* Now, let's set up replication */ USE sandbox_publisher; GO /* Enable publication */ EXECUTE SP_REPLICATIONDBOPTION @dbname = N'sandbox_publisher' , @optname = N'publish' , @VALUE = N'true'; GO /* Create our publication */ EXECUTE SP_ADDPUBLICATION @publication = N'myTestPublication' , @sync_method = N'concurrent' , @description = N'Test Publication' , @status = N'active' , @independent_agent = N'true' , @immediate_sync = N'true' , @retention = 0 GO /* Create our snapshot agent */ EXECUTE SP_ADDPUBLICATION_SNAPSHOT @publication = N'myTestPublication' , @frequency_type = 1; /* Add our table as an article */ EXECUTE SP_ADDARTICLE @publication = N'myTestPublication' , @article = N'myTable' , @source_owner = N'dbo' , @source_object = N'myTable' , @type = N'logbased' , @destination_table = N'myTable' , @destination_owner = N'dbo' , @ins_cmd = N'CALL sp_MSins_dbomyTable' , @del_cmd = N'CALL sp_MSdel_dbomyTable' , @upd_cmd = N'MCALL sp_MSupd_dbomyTable'; GO /* Generate an initial snapshot */ EXECUTE sp_startpublication_snapshot @publication = N'myTestPublication'; /* Create our subscription */ EXECUTE SP_ADDSUBSCRIPTION @publication = N'myTestPublication' , @subscriber = N'YourServerName' , @destination_db = N'sandbox_subscriber'; /* At this point, basic transactional replication should be configured. You should now have a copy of myTable in your subscription database. */ USE sandbox_subscriber; GO /* Let's take a look at fragmentation on the subscription database. */ SELECT avg_fragmentation_in_percent , page_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID(N'MyTable'), NULL , NULL, N'Limited') OPTION (MaxDop 1); /* Results: avg_fragmentation_in_percent page_count ---------------------------- -------------------- 0.29940119760479 334 Less than 1% fragmented! Why is this? During initialization, a copy of the schema and data is sent to the subscriber. The data is sorted before insertion into the destination table. */ USE sandbox_publisher; GO /* Let's insert another 1000 records and see what happens... */ DECLARE @iteration INT; SET @iteration = 0; WHILE @iteration < 1000 BEGIN INSERT INTO sandbox_publisher.dbo.myTable SELECT NewID(), REPLICATE('A', 2000); SET @iteration = @iteration + 1; END SELECT avg_fragmentation_in_percent , page_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID(N'MyTable'), NULL , NULL, N'Limited') OPTION (MaxDop 1); /* Results: avg_fragmentation_in_percent page_count ---------------------------- -------------------- 98.6193293885602 1014 The publisher is still very fragmented (as expected) */ USE sandbox_subscriber; GO SELECT avg_fragmentation_in_percent , page_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID(N'MyTable'), NULL , NULL, N'Limited') OPTION (MaxDop 1); /* Results: avg_fragmentation_in_percent page_count ---------------------------- -------------------- 95.4034729315628 979 But now the subscriber is fragmented too! This is because each individual transaction is replicated, so out-of-order inserts affect both the publishing and the subcribing databases equally. */ /* Finally, let's rebuild our index on the publisher and see what happens */ USE sandbox_publisher; GO ALTER INDEX PK_myTable ON myTable Rebuild; SELECT avg_fragmentation_in_percent , page_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID(N'MyTable'), NULL , NULL, N'Limited') OPTION (MaxDop 1); /* Results: avg_fragmentation_in_percent page_count ---------------------------- -------------------- 0 667 0% fragmentation in the publishing table! What about our subscribing table? */ USE sandbox_subscriber; GO SELECT avg_fragmentation_in_percent , page_count FROM sys.dm_db_index_physical_stats (DB_ID(), OBJECT_ID(N'MyTable'), NULL , NULL, N'Limited') OPTION (MaxDop 1); /* Results: avg_fragmentation_in_percent page_count ---------------------------- -------------------- 95.4034729315628 979 As you can see, the subcribing table was completely unaffected by the defrag activities on the publisher. */
So there you have it, proof that defrag operations are not replicated! Now go defrag your replicated tables.
Oh, and if you need help defragging your indexes, check out my index defrag script (2005, 2008).







