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Fragmentation in SQL Azure!

  • Article

Do you think you are paying for whatever space you are using or you are paying extra?

Most of the people think that they won’t have fragmentation in SQL Azure, but that’s not right SQL Azure is also SQL residing on machines which are present in cloud environment and we have as equal probability of fragmentation in Azure as it is there in on-premise environment.

Let me give you some recent problems on which I worked on,

One of my client was having database of 120GB, he was using federation and having multiple shards, he did split operation which does not involve any data movement but it just creates new shard which will hold data of next month and after Split operation finished his 120GB shard was showing size as 80 GB

Now what happened to those 40 GB?

Number of rows were same there was no data loss so was he paying more than what he was actually storing?

Reason of that data size difference was fragmentation, when he did Split operation SQL Azure engine did rebuild of indexes in background which removed fragmentation and which resulted in reduction in size of database shard.

In another example database size became 350 MB from original 1.5 GB after running query which does rebuild on all indexes of database.(isn’t that great considering how much dollar it would have saved!!!!)

Fragmentation becomes nightmare when we have unique identifier as primary key and large columns like varchar(1000).

Now question comes how to find out whether my database is having fragmentation or not.

 

First let’s find size of all tables in database, most of the people use database size feature of SQL Azure management portal to find out size , I would say instead use SQL Query to find perfect number.

Query:

 

 select obj.name, sum(reserved_page_count) * 8.0 as "size in KB" from sys.dm_db_partition_stats part, sys.objects obj where part.object_id = obj.object_id group by obj.name

 

This query gives size of each table in given database.

Connect to database in concern and run this query.

 

 

 select sum(reserved_page_count) * 8.0 / 1024 as "size in MB" from sys.dm_db_partition_stats

This query gives size of overall database… In case you are not interested in size of each table!

 

Now next task is finding out % of fragmentation…

 

For that also we have query which uses SQL Azure DMV’s to find out % of fragmentation

 

 SELECT
 DB_NAME() AS DBName
 ,OBJECT_NAME(ps.object_id) AS TableName
 ,i.name AS IndexName
 ,ips.index_type_desc
 ,ips.avg_fragmentation_in_percent
 FROM sys.dm_db_partition_stats ps
 INNER JOIN sys.indexes i
 ON ps.object_id = i.object_id
 AND ps.index_id = i.index_id
 CROSS APPLY sys.dm_db_index_physical_stats(DB_ID(), ps.object_id, ps.index_id, null, 'LIMITED') ips
 ORDER BY ps.object_id, ps.index_id

 

Running this query will give you result something like…

 

 

 

So it gives,

1)      Database Name

2)      Table Name

3)      Index Name

4)      Index type

And

5)      % of Fragmentation for that index

 

Any index which shows more than 10% of fragmentation needs rebuild or reorganization of index.

If fragmentation is less than 30% we can go with reorganization of index and if fragmentation is more than 30% then we need to rebuild indexes.

When we rebuild indexes we can do it with ONLINE=ON option which will keep table live while rebuilding happens, it will slow down overall rebuilding process.

 

There is a limitation that when we have any table which has columns which has large data like

varchar(50) varbinary(max) etc , we cannot do rebuild operation with ONLINE=ON .

 

For such table when we do rebuild of indexes table won’t be accessible.

 

Now question is how to do rebuild or reorganization of indexes…?

 

We have query for that also…

 

Query:

 

 DECLARE @TableName varchar(255)
 
 DECLARE TableCursor CURSOR FOR
 (
 SELECT '[' + IST.TABLE_SCHEMA + '].[' + IST.TABLE_NAME + ']' AS [TableName]
 FROM INFORMATION_SCHEMA.TABLES IST
 WHERE IST.TABLE_TYPE = 'BASE TABLE'
 )
 
 OPEN TableCursor
 FETCH NEXT FROM TableCursor INTO @TableName
 WHILE @@FETCH_STATUS = 0
 
 BEGIN
 PRINT('Rebuilding Indexes on ' + @TableName)
 Begin Try
 EXEC('ALTER INDEX ALL ON ' + @TableName + ' REBUILD with (ONLINE=ON)')
 End Try
 Begin Catch
 PRINT('Cannot do rebuild with Online=On option, taking table ' + @TableName+' down for douing rebuild')
 EXEC('ALTER INDEX ALL ON ' + @TableName + ' REBUILD')
 End Catch
 FETCH NEXT FROM TableCursor INTO @TableName
 END
 
 CLOSE TableCursor
 DEALLOCATE TableCursor

 

 

 

You need to connect to database which is having fragmentation problem and run this query, it will do rebuild on all tables present in database.

If it is possible to rebuild indexes while keeping database online it will do that else it will run without online=on option.

 

Output of this query will be something like,

 

 

Once this query finishes executing run query to find database size again and see the magic!

 

Send some gifts if this blog help you in saving some money!!! J

 

Query to find fragmentation on specific database and specific table

 

 

 SELECT a.index_id, name, avg_fragmentation_in_percent
 FROM sys.dm_db_index_physical_stats (DB_ID(N'<Database name>'), OBJECT_ID(N'<table name>'), NULL, NULL, NULL) AS a
 JOIN sys.indexes AS b ON a.object_id = b.object_id AND a.index_id = b.index_id; 
 GO

 

 

Some links which gives more details,

 

https://msdn.microsoft.com/en-us/library/ms189858%28v=sql.105%29.aspx

https://msdn.microsoft.com/en-us/library/ms188917.aspx

https://beyondrelational.com/modules/2/blogs/76/posts/15290/index-fragmentation-in-sql-azure.aspx

Comments

  • Anonymous
    October 01, 2013
    Someone Asked me whether it is possible to rebuild one index at a time, Yes it is possible, i manage to automate it using below query DECLARE @TableName varchar(255) DECLARE @IndexName varchar(255) Declare IndexCursor CURSOR FOR ( select '['+ind.name+']' index_name,'[' + IST.TABLE_SCHEMA + '].[' + IST.TABLE_NAME + ']'  table_name from sys.indexes as ind inner join sys.tables as tab on tab.object_id=ind.object_id inner join information_schema.tables ist on tab.name=ist.TABLE_NAME ) OPEN IndexCursor FETCH NEXT FROM IndexCursor INTO @IndexName,@TableName WHILE @@FETCH_STATUS = 0 BEGIN Begin Try exec('ALTER INDEX '+@Indexname+' ON ' + @TableName + ' REBUILD;') print('Rebuild done for INDEX '+@Indexname+' ON ' + @TableName) End Try Begin Catch print('Rebuild failed for INDEX '+@Indexname+' ON ' + @TableName) End Catch FETCH NEXT FROM IndexCursor INTO @IndexName,@TableName END CLOSE IndexCursor DEALLOCATE IndexCursor

  • Anonymous
    November 20, 2013
    Can you make a rough estimate on how mush time it will take to rebuild the indexes of a 2GB database?

  • Anonymous
    November 20, 2013
    It will actually depend on how fragmented indexes are. I do not have estimates but it could have 30 min to somewhere around 1 hour max. Do online rebuild operation so it will not impact your production(unless it uses up all tempdb space, in which case do index by index rebuild) And then do offline for which online fails.

  • Anonymous
    January 01, 2014
    I tried to run the script ("Now question is how to do rebuild or reorganization of indexes") but after a while it throw: Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding. Any way to fix the timeout?

  • Anonymous
    January 01, 2014
    Nevermind, The timeout was the client (azure's SQL database manager), so I run the script per table ALTER INDEX ALL ON dbo.SMSMessages REBUILD;

  • Anonymous
    January 02, 2014
    G You can also use per index fragmentation mentioned in first comment.

  • Anonymous
    February 18, 2014
    The comment has been removed

  • Anonymous
    February 19, 2014
    The comment has been removed

  • Anonymous
    March 10, 2015
    I've been fighting with a DotNetNuke install hosted on Azure for a while. We've been testing as there's a lot we like about Azure but the performance when editing DotNetNuke has caused us to go a different route...but that's another..

  • Anonymous
    September 10, 2015
    Hi beyondrelational.com/.../index-fragmentation-in-sql-azure.aspx Blog is saying FILLFACTOR (it is defaulted to 100%, and cannot be changed) REORGANIZE (we have REBUILD only) SORT_IN_TEMPDB DATA_COMPRESSION are won't work on SQL Azure ..Is it true ?

  • Anonymous
    September 10, 2015
    The comment has been removed

  • Anonymous
    January 24, 2016
    Wow - thanks for this post! - I was puzzling why I took a copy of our Azure db and one of the tables used less than 1/5 of the space on Azure. Turns out index fragmentation was 99%!