DBCC SHOWCONTIG (Transact-SQL)

Artikel
12/03/2008

Displays fragmentation information for the data and indexes of the specified table or view.

Important

This feature will be removed in a future version of Microsoft SQL Server. Avoid using this feature in new development work, and plan to modify applications that currently use this feature. Use sys.dm_db_index_physical_stats instead.

Topic link icon Transact-SQL Syntax Conventions

Syntax

DBCC SHOWCONTIG 
[ ( 
    { table_name | table_id | view_name | view_id } 
    [ , index_name | index_id ] 
) ] 
    [ WITH 
        { 
         [ , [ ALL_INDEXES ] ] 
         [ , [ TABLERESULTS ] ] 
         [ , [ FAST ] ]
         [ , [ ALL_LEVELS ] ] 
         [ NO_INFOMSGS ]
         }
    ]

Arguments

table_name | table_id | view_name | view_id
Is the table or view for which to check fragmentation information. If not specified, all tables and indexed views in the current database are checked. To obtain the table or view ID, use the OBJECT_ID function.

index_name | index_id
Is the index for which to check fragmentation information. If not specified, the statement processes the base index for the specified table or view. To obtain the index ID, use the sys.indexes catalog view.

WITH
Specifies options for the type of information returned by the DBCC statement.

FAST
Specifies whether to perform a fast scan of the index and output minimal information. A fast scan does not read the leaf or data level pages of the index.

ALL_INDEXES
Displays results for all the indexes for the specified tables and views, even if a particular index is specified.

TABLERESULTS
Displays results as a rowset, with additional information.

ALL_LEVELS
Maintained for backward compatibility only. Even if ALL_LEVELS is specified, only the index leaf level or table data level is processed.

NO_INFOMSGS
Suppresses all informational messages that have severity levels from 0 through 10.

Result Sets

The following table describes the information in the result set.

Statistic

Description

Pages Scanned

Number of pages in the table or index.

Extents Scanned

Number of extents in the table or index.

Extent Switches

Number of times the DBCC statement moved from one extent to another while the statement traversed the pages of the table or index.

Avg. Pages per Extent

Number of pages per extent in the page chain.

Scan Density [Best Count: Actual Count]

Is a percentage. It is the ratio Best Count to Actual Count. This value is 100 if everything is contiguous; if this value is less than 100, some fragmentation exists.

Best Count is the ideal number of extent changes if everything is contiguously linked. Actual Count is the actual number of extent changes.

Logical Scan Fragmentation

Percentage of out-of-order pages returned from scanning the leaf pages of an index. This number is not relevant to heaps. An out-of-order page is a page for which the next physical page allocated to the index is not the page pointed to by the next-page pointer in the current leaf page.

Extent Scan Fragmentation

Percentage of out-of-order extents in scanning the leaf pages of an index. This number is not relevant to heaps. An out-of-order extent is one for which the extent that contains the current page for an index is not physically the next extent after the extent that contains the previous page for an index.

ms175008.note(en-US,SQL.90).gif Note:

This number is meaningless when the index spans multiple files.

Avg. Bytes Free per Page

Average number of free bytes on the pages scanned. The larger the number, the less full the pages are. Lower numbers are better if the index will not have many random inserts. This number is also affected by row size; a large row size can cause a larger number.

Avg. Page density (full)

Average page density, as a percentage. This value takes into account row size. Therefore, the value is a more accurate indication of how full your pages are. The larger the percentage, the better.

When table_id and FAST are specified, DBCC SHOWCONTIG returns a result set with only the following columns.

Pages Scanned
Extent Switches
Scan Density [Best Count:Actual Count]
Extent Scan Fragmentation
Logical Scan Fragmentation

When TABLERESULTS is specified, DBCC SHOWCONTIG returns the following columns and also the nine columns described in the previous table.

Statistic

Description

Object Name

Name of the table or view processed.

ObjectId

ID of the object name.

IndexName

Name of the index processed. Is NULL for a heap.

IndexId

ID of the index. Is 0 for a heap.

Level

Level of the index. Level 0 is the leaf, or data, level of the index.

Level is 0 for a heap.

Pages

Number of pages that make up that level of the index or whole heap.

Rows

Number of data or index records at that level of the index. For a heap, this value is the number of data records in the whole heap.

For a heap, the number of records returned by this function might not match the number of rows that are returned by running a SELECT COUNT(*) against the heap. This is because a row may contain multiple records. For example, under some update situations, a single heap row may have a forwarding record and a forwarded record as a result of the update operation. Also, most large LOB rows are split into multiple records in LOB_DATA storage.

MinimumRecordSize

Minimum record size in that level of the index or whole heap.

MaximumRecordSize

Maximum record size in that level of the index or whole heap.

AverageRecordSize

Average record size in that level of the index or whole heap.

ForwardedRecords

Number of forwarded records in that level of the index or whole heap.

Extents

Number of extents in that level of the index or whole heap.

ExtentSwitches

Number of times the DBCC statement moved from one extent to another while the statement traversed the pages of the table or index.

AverageFreeBytes

AveragePageDensity

Average page density, as a percentage. This value takes into account row size. Therefore, the value is a more accurate indication of how full your pages are. The larger the percentage, the better.

ScanDensity

Is a percentage. It is the ratio BestCount to ActualCount. This value is 100 if everything is contiguous; if this value is less than 100, some fragmentation exists.

BestCount

Is the ideal number of extent changes if everything is contiguously linked.

ActualCount

Is the actual number of extent changes.

LogicalFragmentation

ExtentFragmentation

ms175008.note(en-US,SQL.90).gif Note:

This number is meaningless when the index spans multiple files.

When WITH TABLERESULTS and FAST are specified, the result set is the same as when WITH TABLERESULTS is specified, except the following columns will have null values:

Rows	Extents
MinimumRecordSize	AverageFreeBytes
MaximumRecordSize	AveragePageDensity
AverageRecordSize	ExtentFragmentation
ForwardedRecords

Remarks

The DBCC SHOWCONTIG statement traverses the page chain at the leaf level of the specified index when index_id is specified. If only table_id is specified or if index_id is 0, the data pages of the specified table are scanned. The operation only requires an intent-shared (IS) table lock. This way all updates and inserts can be performed, except those that require an exclusive (X) table lock. This allows for a tradeoff between speed of execution and no reduction in concurrency against the number of statistics returned. However, if the command is being used only to gauge fragmentation, we recommend using the WITH FAST option for optimal performance. A fast scan does not read the leaf or data level pages of the index. The WITH FAST option does not apply to a heap.

Changes in SQL Server 2005

The algorithm for calculating fragmentation is more precise in SQL Server 2005 than in SQL Server 2000. As a result, the fragmentation values will appear higher. For example, in SQL Server 2000, a table is not considered fragmented if it has page 11 and page 13 in the same extent but not page 12. However, to access these two pages would require two physical I/O operations, so this is counted as fragmentation in SQL Server 2005.

DBCC SHOWCONTIG does not display data with ntext, text, and image data types. This is because text indexes (index ID 255 in SQL Server 2000) that store text and image data no longer exists in SQL Server 2005. For more information about index ID 255, see sys.sysindexes (Transact-SQL).

Also, DBCC SHOWCONTIG does not support some new features in SQL Server 2005. For example:

If the specified table or index is partitioned, DBCC SHOWCONTIG only displays the first partition of the specified table or index.
DBCC SHOWCONTIG does not display row-overflow storage information and other new off-row data types, such as nvarchar(max), varchar(max), varbinary(max), and xml.

All new features in SQL Server 2005 are fully supported by the sys.dm_db_index_physical_stats dynamic management view.

Table Fragmentation

DBCC SHOWCONTIG determines whether the table is heavily fragmented. Table fragmentation occurs through the process of data modifications (INSERT, UPDATE, and DELETE statements) made against the table. Because these modifications are not ordinarily distributed equally among the rows of the table, the fullness of each page can vary over time. For queries that scan part or all of a table, such table fragmentation can cause additional page reads. This hinders parallel scanning of data.

When an index is heavily fragmented, the following choices are available for reducing fragmentation:

Drop and re-create a clustered index.
Re-creating a clustered index reorganizes the data, and causes full data pages. The level of fullness can be configured by using the FILLFACTOR option in CREATE INDEX. The drawbacks of this method are that the index is offline during the drop or re-create cycle, and that the operation is atomic. If the index creation is interrupted, the index is not re-created.
Reorder the leaf-level pages of the index in a logical order.
Use ALTER INDEX…REORGANIZE to reorder the leaf-level pages of the index in a logical order. Because this operation is an online operation, the index is available when the statement is running. The operation is also interruptible without loss of completed work. The drawback of this method is that the method does not do as good a job of reorganizing the data as a clustered index drop or re-create operation.
Rebuild the index.
Use ALTER INDEX with REBUILD to rebuild the index. For more information, see ALTER INDEX (Transact-SQL).

The Avg. Bytes free per page and Avg. Page density (full) statistic in the result set indicate the fullness of index pages. The Avg. Bytes free per page number should be low and the Avg. Page density (full) number should be high for an index that will not have many random inserts. Dropping and re-creating an index with the FILLFACTOR option specified can improve the statistics. Also, ALTER INDEX with REORGANIZE will compact an index, taking into account its FILLFACTOR, and will improve the statistics.

Note

An index that has many random inserts and very full pages will have an increased number of page splits. This causes more fragmentation.

The fragmentation level of an index can be determined in the following ways:

By comparing the values of Extent Switches and Extents Scanned.
The value of Extent Switches should be as close as possible to that of Extents Scanned. This ratio is calculated as the Scan Density value. This value should be as high as possible, and can be improved by reducing index fragmentation.

Note

This method does not work if the index spans multiple files.
By understanding Logical Scan Fragmentation and Extent Scan Fragmentation values.
Logical Scan Fragmentation and, to a lesser extent, Extent Scan Fragmentation values are the best indicators of the fragmentation level of a table. Both these values should be as close to zero as possible, although a value from 0 through 10 percent may be acceptable.

Note

The Extent Scan Fragmentation value will be high if the index spans multiple files. To reduce these values, you must reduce the index fragmentation.

Permissions

User must own the table, or be a member of the sysadmin fixed server role, the db_owner fixed database role, or the db_ddladmin fixed database role.

Examples

A. Displaying fragmentation information for a table

The following example displays fragmentation information for the Employee table.

USE AdventureWorks;
GO
DBCC SHOWCONTIG ("HumanResources.Employee");
GO

B. Using OBJECT_ID to obtain the table ID and sys.indexes to obtain the index ID

The following example uses OBJECT_ID and the sys.indexes catalog view to obtain the table ID and index ID for the AK_Product_Name index of the Production.Product table in the AdventureWorks database.

USE AdventureWorks;
GO
DECLARE @id int, @indid int
SET @id = OBJECT_ID('Production.Product')
SELECT @indid = index_id 
FROM sys.indexes
WHERE object_id = @id 
   AND name = 'AK_Product_Name'
DBCC SHOWCONTIG (@id, @indid);
GO

C. Displaying an abbreviated result set for a table

The following example returns an abbreviated result set for the Product table in the AdventureWorks database.

USE AdventureWorks;
GO
DBCC SHOWCONTIG ("Production.Product", 1) WITH FAST;
GO

D. Displaying the full result set for every index on every table in a database

The following example returns a full table result set for every index on every table in the AdventureWorks database.

USE AdventureWorks;
GO
DBCC SHOWCONTIG WITH TABLERESULTS, ALL_INDEXES;
GO

E. Using DBCC SHOWCONTIG and DBCC INDEXDEFRAG to defragment the indexes in a database

The following example shows a simple way to defragment all indexes in a database that is fragmented above a declared threshold.

/*Perform a 'USE <database name>' to select the database in which to run the script.*/
-- Declare variables
SET NOCOUNT ON;
DECLARE @tablename varchar(255);
DECLARE @execstr   varchar(400);
DECLARE @objectid  int;
DECLARE @indexid   int;
DECLARE @frag      decimal;
DECLARE @maxfrag   decimal;

-- Decide on the maximum fragmentation to allow for.
SELECT @maxfrag = 30.0;

-- Declare a cursor.
DECLARE tables CURSOR FOR
   SELECT TABLE_SCHEMA + '.' + TABLE_NAME
   FROM INFORMATION_SCHEMA.TABLES
   WHERE TABLE_TYPE = 'BASE TABLE';

-- Create the table.
CREATE TABLE #fraglist (
   ObjectName char(255),
   ObjectId int,
   IndexName char(255),
   IndexId int,
   Lvl int,
   CountPages int,
   CountRows int,
   MinRecSize int,
   MaxRecSize int,
   AvgRecSize int,
   ForRecCount int,
   Extents int,
   ExtentSwitches int,
   AvgFreeBytes int,
   AvgPageDensity int,
   ScanDensity decimal,
   BestCount int,
   ActualCount int,
   LogicalFrag decimal,
   ExtentFrag decimal);

-- Open the cursor.
OPEN tables;

-- Loop through all the tables in the database.
FETCH NEXT
   FROM tables
   INTO @tablename;

WHILE @@FETCH_STATUS = 0
BEGIN;
-- Do the showcontig of all indexes of the table
   INSERT INTO #fraglist 
   EXEC ('DBCC SHOWCONTIG (''' + @tablename + ''') 
      WITH FAST, TABLERESULTS, ALL_INDEXES, NO_INFOMSGS');
   FETCH NEXT
      FROM tables
      INTO @tablename;
END;

-- Close and deallocate the cursor.
CLOSE tables;
DEALLOCATE tables;

-- Declare the cursor for the list of indexes to be defragged.
DECLARE indexes CURSOR FOR
   SELECT ObjectName, ObjectId, IndexId, LogicalFrag
   FROM #fraglist
   WHERE LogicalFrag >= @maxfrag
      AND INDEXPROPERTY (ObjectId, IndexName, 'IndexDepth') > 0;

-- Open the cursor.
OPEN indexes;

-- Loop through the indexes.
FETCH NEXT
   FROM indexes
   INTO @tablename, @objectid, @indexid, @frag;

WHILE @@FETCH_STATUS = 0
BEGIN;
   PRINT 'Executing DBCC INDEXDEFRAG (0, ' + RTRIM(@tablename) + ',
      ' + RTRIM(@indexid) + ') - fragmentation currently '
       + RTRIM(CONVERT(varchar(15),@frag)) + '%';
   SELECT @execstr = 'DBCC INDEXDEFRAG (0, ' + RTRIM(@objectid) + ',
       ' + RTRIM(@indexid) + ')';
   EXEC (@execstr);

   FETCH NEXT
      FROM indexes
      INTO @tablename, @objectid, @indexid, @frag;
END;

-- Close and deallocate the cursor.
CLOSE indexes;
DEALLOCATE indexes;

-- Delete the temporary table.
DROP TABLE #fraglist;
GO

Change History

Release	History
15 September 2007	Changed content: Corrected the locking information in the Remarks section. In SQL Server 2005, this command uses only an intent-shared (IS) table lock, not an S lock as previously stated. Clarified the definition of the Rows column for heaps.
17 July 2006	New content: Added information about the algorithm for calculating fragmentation in the section "Changes in SQL Server 2005."
5 December 2005	Changed content: Corrected the description of LogicalFragmentation.

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