Database Console Commands – DBCC

DBCC statements are grouped into the following categories.

·        Maintenance

o       Maintenance tasks on a database, index, or filegroup

·        Miscellaneous

o       Miscellaneous tasks such as enabling trace flags or removing a DLL from memory

·        Informational

o       Tasks that gather and display various types of information

·        Validation

o       Validation operations on a database, table, index, catalog, filegroup, or allocation of database pages.

 

"You get an error in the logs indicating that you have a torn page or checksum error. What do you do?"

Run DBCC CHECKDB with one of the repair options

So here is how to use DBCC CHECKDB, and what to do when you have a torn or corrupt page.

DBCC CHECKDB ('DB Name') WITH NO_INFOMSGS

NO_INFOMSGS prevents an excessive number of informational messages from being generated.

This is the syntax should aim to use as it performs all integrity checks.

This may take a long time on large databases and you may want to specify the PHYSICAL_ONLY option. This checks physical on-disk structures, but omits the internal logical checks. The syntax is:

DBCC CHECKDB ('DB Name') WITH  PHYSICAL_ONLY

It Has Found A Problem - What Do I Do?

A non-clustered index can be dropped and rebuilt but actual data, such as a clustered index, cannot.

Let's look at how you investigate which pages are affected and what type of data is affected. Look at the output from DBCC CHECKDB.

The first thing to do is check if it is data in a heap, in a clustered index, or in a non-clustered index. Error is an index ID 0.

Then examine the page (1:94299 in database 'yourdb') as follows:

DBCC TRACEON (3604, -1)
GO
DBCC PAGE('yourdb', 1, 94299, 3)
GO

In the output you will see something like:
Metadata: IndexId = n

1. If n is greater than 1 it is a non-clustered index and can safely be dropped and recreated.

2. If n is 0 or 1 you have data corruption and need to perform one of the options described below.

1. Restoring from a backup

2. Automatic Repair Options

                                                              i.      REPAIR_REBUILD

                                                            ii.      REPAIR_ALLOW_DATA_LOSS

Restoring from a backup

If the recovery model is FULL (or BULK_LOGGED, with some limitations), you can backup the tail of the log, perform a restore (with norecovery) from the last clean full backup, followed by subsequent log backups and finally the tail of the log.

      If only a few pages are affected you have the option of selectively restoring only the bad pages, as follows:

RESTORE DATABASE yourdb PAGE = '1:94299'
FROM DISK = 'C:\yourdb.bak'
WITH NORECOVERY

If the recovery model is simple you don't have that option, and have to accept that a restore from the last full backup will result in subsequent transactions being lost.

            In this case, or if you have no backups at all, you may decide that an automatic repair is the only option.

Automatic Repair Options

First let me emphasize the importance of running a backup BEFORE you go any further.

Have a look at the output of the original CHECKDB. It will specify the minimum repair level.

REPAIR_REBUILD

If the minimum repair level is REPAIR_REBUILD you have been lucky.
The syntax is

DBCC CHECKDB('DB Name', REPAIR_REBUILD)

REPAIR_ALLOW_DATA_LOSS

This attempts to repair all errors. Sometimes the only way to repair an error is to deallocate the affected page and modify page links so that it looks like the page never existed.

This has the desired effect of restoring the database's structural integrity but means that something has been deleted (hence the ALLOW_DATA_LOSS).

There are likely to be issues with referential integrity, not to mention the important data that may now be missing.

DBCC CHECKDB('DB Name', REPAIR_ALLOW_DATA_LOSS)

Make sure you run DBCC CHECKCONSTRAINTS afterwards so you are aware of referential integrity issues and can take the appropriate action.

And Finally

Only use the automatic repair options as a last resort, and with full understanding of the damage this may do.

Just as important is that regular backups are an essential part of a DBA's responsibilities, and you should use the FULL recovery model with regular log backups for all but the most trivial databases.

DBCC CHECKDB is a powerful tool, but also very dangerous in the wrong hands.

Maybe instead of adding the REPAIR_ALLOW_DATA_LOSS option, Microsoft should have created a separate DBCC command called:

DBCC DELETE_DATA_BECAUSE_I_COULDNT_BE_BOTHERED_TO_TAKE_A_BACKUP

Types of Database Console Commands are follows:

1.DBCC CHECKALLOC

DBCC CHECKALLOC checks page usage and allocation in the database. Use this command if allocation errors are found for the database. If you run DBCC CHECKDB, you do not need to run DBCC CHECKALLOC, as DBCC CHECKDB includes the same checks (and more) that DBCC CHECKALLOC performs.


2.DBCC CHECKCATALOG

This command checks for consistency in and between system tables. This command is not executed within the DBCC CHECKDB command, so running this command weekly is recommended.

3.DBCC CHECKCONSTRAINTS

DBCC CHECKCONSTRAINTS alerts you to any CHECK or constraint violations.
Use it if you suspect that there are rows in your tables that do not meet the constraint or CHECK constraint rules.

4.DBCC CHECKDB

A very important DBCC command, DBCC CHECKDB should run on your SQL Server instance on at least a weekly basis. Although each release of SQL Server reduces occurrences of integrity or allocation errors, they still do happen. DBCC CHECKDB includes the same checks as DBCC CHECKALLOC and DBCC CHECKTABLE. DBCC CHECKDB can be rough on concurrency, so be sure to run it at off-peak times.

5.DBCC CHECKTABLE

DBCC CHECKTABLE is almost identical to DBCC CHECKDB, except that it is performed at the table level, not the database level. DBCC CHECKTABLE verifies index and data page links, index sort order, page pointers, index pointers, data page integrity, and page offsets. DBCC CHECKTABLE uses schema locks by default, but can use the TABLOCK option to acquire a shared table lock. CHECKTABLE also performs object checking using parallelism by default (if on a multi-CPU system).

6.DBCC CHECKFILEGROUP

DBCC CHECKFILEGROUP works just like DBCC CHECKDB, only DBCC CHECKFILEGROUP checks the specified filegroup for allocation and structural issues. If you have a very large database (this term is relative, and higher end systems may be more apt at performing well with multi-GB or TB systems ) , running DBCC CHECKDB may be time-prohibitive.
If your database is divided into user defined filegroups, DBCC CHECKFILEGROUP will allow you to isolate your integrity checks, as well as stagger them over time.

7.DBCC CHECKIDENT

DBCC CHECKIDENT returns the current identity value for the specified table, and allows you to correct the identity value if necessary.

8.DBCC DBREINDEX

If your database allows modifications and has indexes, you should rebuild your indexes on a regular basis. The frequency of your index rebuilds depends on the level of database activity, and how quickly your database and indexes become fragmented. DBCC DBREINDEX allows you to rebuild one or all indexes for a table. Like DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKALLOC, running DBREINDEX during peak activity times can significantly reduce concurrency.

9.DBCC INDEXDEFRAG

Microsoft introduced the excellent DBCC INDEXDEFRAG statement beginning with SQL Server 2000. This DBCC command, unlike DBCC DBREINDEX, does not hold long term locks on indexes.

Use DBCC INDEXDEFRAG for indexes that are not very fragmented, otherwise the time this operation takes will be far longer then running DBCC DBREINDEX.

In spite of it's ability to run during peak periods, DBCC INDEXDEFRAG has had limited effectiveness compared to DBCC DBREINDEX (or drop/create index).

10.DBCC INPUTBUFFER

The DBCC INPUTBUFFER command is used to view the last statement sent by the client connection to SQL Server. When calling this DBCC command, you designate the SPID to examine. (SPID is the process ID, which you can get from viewing current activity in Enterprise Manager or executing sp_who. )

11.DBCC OPENTRAN

DBCC OPENTRAN is a Transact-SQL command that is used to view the oldest running transaction for the selected database.

The DBCC command is very useful for troubleshooting orphaned connections (connections still open on the database but disconnected from the application or client), and identification of transactions missing a COMMIT or ROLLBACK.

This command also returns the oldest distributed and undistributed replicated transactions, if any exist within the database. If there are no active transactions, no data will be returned.

If you are having issues with your transaction log not truncating inactive portions, DBCC OPENTRAN can show if an open transaction may be causing it.

12.DBCC PROCCACHE

You may not use this too frequently, however it is an interesting DBCC command to execute periodically, particularly when you suspect you have memory issues. DBCC PROCCACHE provides information about the size and usage of the SQL Server procedure cache.

13.DBCC SHOWCONTIG

The DBCC SHOWCONTIG command reveals the level of fragmentation for a specific table and its indices. This DBCC command is critical to determining if your table or index has internal or external fragmentation. Internal fragmentation concerns how full an 8K page is.

When a page is underutilized, more I/O operations may be necessary to fulfill a query request than if the page was full, or almost full.

External fragmentation concerns how contiguous the extents are. There are eight 8K pages per extent, making each extent 64K. Several extents can make up the data of a table or index.

If the extents are not physically close to each other, and are not in order, performance could diminish.

14.DBCC SHRINKDATABASE

DBCC SHRINKDATABASE shrinks the data and log files in your database.
Avoid executing this command during busy periods in production, as it has a negative impact on I/O and user concurrency.

Also remember that you cannot shrink a database past the target percentage specified, shrink smaller than the model database, shrink a file past the original file creation size, or shrink a file size used in an ALTER DATABASE statement.

15.DBCC SHRINKFILE

DBCC SHRINKFILE allows you to shrink the size of individual data and log files. (Use sp_helpfile to gather database file ids and sizes).

16. DBCC TRACEOFF, TRACEON, TRACESTATUS

Trace flags are used within SQL Server to temporarily enable or disable specific SQL Server instance characteristics. Traces are enabled using the DBCC TRACEON command, and disabled using DBCC TRACEOFF. DBCC TRACESTATUS is used to displays the status of trace flags. You'll most often see TRACEON used in conjunction with deadlock logging (providing more verbose error information).

17.DBCC USEROPTIONS

Execute DBCC USEROPTIONS to see what user options are in effect for your specific user connection. This can be helpful if you are trying to determine if you current user options are inconsistent with the database options.