Sunday, May 8, 2011

Reclaim Space using DBCC CLEANTABLE

Sweep

Image by The Real Estreya via Flickr

I just read Pradeep Adiga's excellent post on using DBCC CLEANTABLE to reclaim space left over after a column with a variable length type has been dropped from a table.

According to Books OnLine, the variable length data types are VARCHAR, NVARCHAR, VARCHAR(MAX), NVARCHAR(MAX), VARBINARY, VARBINARY(MAX), TEXT, NTEXT, IMAGE, SQL_VARIANT, and XML. If you drop one of these type columns from a table, SQL Server doesn't release the space it occupied; I imagine this is a speed optimization.

Thus inspired, here's a little script to run DBCC CLEANTABLE on all the user tables in the current database. You might be thinking, "How often does a column get dropped from a table, really?" The answer of course is, "It depends." On one huge (and abused) database I ran this script on, I gained 8 GB of space!

-----------------------------------------------------------------------------
-- Reclaim space from dropped variable length columns and text columns.      
-----------------------------------------------------------------------------

RAISERROR('Reclaiming space from dropped variable length columns and text columns...', 10, 1) WITH NOWAIT

DECLARE @TableName  sysname       = ''
DECLARE @SchemaName sysname       = ''

DECLARE cur CURSOR FOR 
    SELECT SCHEMA_NAME(schema_id)   AS 'SchemaName' 
         , name                     AS 'TableName'
      FROM sys.tables
     WHERE is_ms_shipped = 0
       AND type_desc     = 'USER_TABLE'
  ORDER BY 'SchemaName', 'TableName'

OPEN cur
FETCH NEXT FROM cur INTO @SchemaName, @TableName        

WHILE @@FETCH_STATUS = 0
BEGIN
    SET @TableName = @SchemaName + '.' + @TableName
    RAISERROR('   Examining table %s', 10, 1, @TableName) WITH NOWAIT
    DBCC CLEANTABLE(0, @TableName, 100000) WITH NO_INFOMSGS
    FETCH NEXT FROM cur INTO @SchemaName, @TableName        
END

CLOSE cur
DEALLOCATE cur

RAISERROR('Done.', 10, 1) WITH NOWAIT

Note that we specify a "batch_size" value of 100,000 rows per transaction to keep the logfile small while processing, and to keep the table locks (!) short. The appropriate value for a given system will require some experimentation: for mine, it stopped getting faster at 100,000 rows. Also, BOL points out that this script does not need to be run on a regular basis, just when a variable length or text column is dropped: but how often that actually is depends on your environment.

1 comments:

ast13 said...

Excellent! Thank you! This saved me lots of time.

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