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SQL Server 6.5: Some useful undocumented DBCC commands

Alexander Chigrik
chigrik@mssqlcity.com


Introduction
Undocumented DBCC commands:
  • DBCC allocdump
  • DBCC bhash
  • DBCC buffer
  • DBCC bytes
  • DBCC dbinfo
  • DBCC dbtable
  • DBCC delete_row
  • DBCC des
  • DBCC extentchain
  • DBCC extentcheck
  • DBCC extentdump
  • DBCC extentzap
  • DBCC findnotfullextents
  • DBCC help
  • DBCC ind
  • DBCC locateindexpgs
  • DBCC lock
  • DBCC log
  • DBCC page
  • DBCC pglinkage
  • DBCC procbuf
  • DBCC prtipage
  • DBCC pss
  • DBCC rebuildextents
  • DBCC resource
  • DBCC show_bucket
  • DBCC tab
  • DBCC undo
  • Literature


    Introduction

    In this article, I want to tell you about some useful undocumented DBCC commands, and how you can use these commands in SQL Server 6.5 for administering and monitoring.

    DBCC is an abbreviation for Database Console Command. DBCC commands are generally used to check the physical and logical consistency of a database, although they are also used for a variety of miscellaneous tasks, as you will see here.

    Note, the command:

    DBCC TRACEON (3604)

    is issued before each of the following DBCC examples in order to better demonstrate the effects of the command by displaying a trace of the output of the DBCC command. It is not actually required to run the DBCC commands examined below. If you run any of the DBCC commands below without the above option, the command runs, but you don't see what it is doing.

    Undocumented DBCC commands

    Here you can find some useful SQL Server 6.5 undocumented DBCC commands.

    DBCC allocdump

    This command can be used to display all the extents on an allocation page.

    Syntax:

    DBCC allocdump( dbid, page )

    where
    
      dbid - is the database id
      page - is the allocation page number
    
    Example:

    DBCC TRACEON (3604)
    DECLARE @dbid int, @pageid int
    SELECT @dbid = DB_ID('pubs')
    SELECT @pageid = first FROM sysindexes WHERE
      id = object_id('titleauthor') AND indid = 1
    DBCC allocdump(@dbid, @pageid)
    

    DBCC bhash

    This command can check the integrity of the buffer hash table, and optionally print it.

    Syntax:

    DBCC bhash( { print_bufs | no_print }, bucket_limit )

    where 
    
      print_bufs   - display all buffers (default)  
      no_print     - display only buffers with problems  
      bucket_limit - number of buffers allowed in a bucket (default = 0)
    
    Example:

    DBCC TRACEON (3604)
    DBCC bhash
    

    DBCC buffer

    This command can be used to display buffer headers and pages from the buffer cache.

    Syntax:

    DBCC buffer([dbid|dbname][,objid|objname][,nbufs], printopt = {0|1|2},buftype)

    where 
    
      dbid|dbname   - database name or database ID
                      (0 will show data for all databases)
      objid|objname - object ID or object name
                      (0 will show data for all objects)
      nbufs         - number of buffers to examine
      printopt      - print option
                      0  print out only the buffer header and page header (default)
                      1  print out each row separately and the offset table
                      2  print out each row as a whole and the offset table
      buftype       - buffer type to print
    
    Example:

    DBCC TRACEON (3604)
    dbcc buffer(0,'sysobjects')
    

    DBCC bytes

    This command can be used to dump out bytes from a specific address.

    Syntax:

    DBCC bytes( startaddress, length )

    where
    
      startaddress  - starting address to dump  
      length        - number of bytes to dump  
    
    Example:

    DBCC TRACEON (3604)
    dbcc bytes (1000000, 100)
    

    DBCC DBINFO

    Displays DBINFO structure for the specified database.

    Syntax:

    DBCC DBINFO [( dbname )]

    where
    
      dbname - is the database name.
    
    Example:

    DBCC TRACEON (3604)
    DBCC DBINFO (pubs)
    

    DBCC DBTABLE

    This command displays the contents of the DBTABLE structure.

    Syntax:

    DBCC DBTABLE ({dbid|dbname})

    where
    
      dbid|dbname  - database name or database ID
    
    Example:

    DBCC TRACEON (3604)
    DBCC DBTABLE (pubs)
    
    The DBTABLE structure has an output parameter called dbt_open. This parameter keeps track of how many users are in the database.

    Look at here for more details:
    FIX: Database Usage Count Does Not Return to Zero

    DBCC delete_row

    This command can be used to delete an index or data row, by either a row number or an offset on a page. DBCC delete_row is a nonlogged command, so you can delete a particular row without placed it into the transaction log. Use this command at your own risk!

    Syntax:

    DBCC delete_row ( dbid|dbname, page, delete_by_row, rownum )

    where
    
      dbid|dbname    - database ID or database name
      page           - logical page number
      delete_by_row  - how delete: by row or by offset
                       (1 - the next parameter is row number)
                       (0 - the next parameter is offset on the page)
      rownum         - row number or offset
    
    Example:

    SET NOCOUNT ON
    GO
    USE pubs
    GO
    DBCC TRACEON (3604)
    GO
    DECLARE @pgid int
    SELECT COUNT(*) FROM titleauthor
    SELECT @pgid = first FROM sysindexes WHERE
      id = object_id('titleauthor') AND indid = 1
    SELECT xactid AS TRAN_ID, op AS LOG_RECORD FROM syslogs
    DBCC delete_row (pubs, @pgid, 'row', 1)
    SELECT xactid AS TRAN_ID, op AS LOG_RECORD FROM syslogs
    SELECT COUNT(*) FROM titleauthor
    

    DBCC des

    This command used to print the contents of the specified DES (descriptor).

    Syntax:

    DBCC des( [ dbid|dbname ][, objid ] )

    where
    
      dbid|dbname - database ID or database name
      objid       - object ID
    
    Example:

    DBCC TRACEON (3604)
    DBCC DES
    

    DBCC extentchain

    This command displays extent header info for all extents in use by the specified object.

    Syntax:

    DBCC extentchain(dbid,objid,indexid,sort={1|0},display={1|0} [,order={1|0}])

    where
    
      dbid     - database ID     
      objid    - object ID     
      indexid  - index ID     
      sort     - report on state of the sort bit
                 (0  don't include this info)
                 (1  do report on sort bit status)
      display  - what to report
                 (0  display the count only)
                 (1  display the extents)
      order    - (optional) order to follow the chain
                 (0  descending, 1  ascending)
    
    Example:

    DBCC TRACEON (3604)
    DECLARE @dbid int, @objectid int
    SELECT @dbid = DB_ID('pubs')
    SELECT @objectid = object_id('authors')
    dbcc extentchain(@dbid,@objectid,0,0,0,0)
    

    DBCC extentcheck

    This command has the same output as extentchain, and examines all extents on allocation pages for the specified object.

    Syntax:

    DBCC extentcheck( dbid, objid, indexid, sort = {1|0} )

    where
    
      dbid    - database ID    
      objid   - object ID    
      indexid - index ID    
      sort    - state of the sort bit
                (0  don't report this, 1  report sort bit status)
    
    Example:

    DBCC TRACEON (3604)
    DECLARE @dbid int, @objectid int
    SELECT @dbid = DB_ID('pubs')
    SELECT @objectid = object_id('authors')
    dbcc extentcheck(@dbid,@objectid,0,1)
    

    DBCC extentdump

    This command displays an extent dump.

    Syntax:

    DBCC extentdump( dbid, page )

    where
    
      dbid  - database ID  
      page  - the number of a page controlled by the extent in question
    
    Example:

    DBCC TRACEON (3604)
    DECLARE @dbid int
    SELECT @dbid = DB_ID('pubs')
    DBCC extentdump(@dbid, 1)
    

    DBCC extentzap

    This command can be used to clear all extents matching the parameter values.

    Syntax:

    DBCC extentzap( dbid, objid, indexid, sort )

    where
    
      dbid    - database ID    
      objid   - object ID    
      indexid - index ID    
      sort    - state of the sort bit
    
    Example:

    DBCC TRACEON (3604)
    DECLARE @dbid int, @objectid int
    SELECT @dbid = DB_ID('pubs')
    SELECT @objectid = object_id('authors')
    DBCC extentzap(@dbid,@objectid,0,0)
    

    DBCC findnotfullextents

    This command shows the extent id of extents allocated to the specified objid that are not full. If the objid is specified, then an indexid can be specified. If objid is all, then all database tables are displayed.

    Syntax:

    DBCC findnotfullextents( dbid,objid,indexid, sort = {1|0} )

    where
    
      dbid    - database ID    
      objid   - object ID    
      indexid - index ID    
      sort    - state of the sort bit
    
    Example:

    DBCC TRACEON (3604)
    DECLARE @dbid int, @objectid int
    SELECT @dbid = DB_ID('pubs')
    SELECT @objectid = object_id('authors')
    DBCC findnotfullextents(@dbid,@objectid,0,0)
    

    DBCC HELP

    DBCC HELP returns syntax information for the specified DBCC statement.

    Syntax:

    DBCC HELP ('dbcc_statement' | @dbcc_statement_var | '?')

    This is the example:

    DBCC TRACEON (3604)
    DECLARE @dbcc_stmt sysname
    SELECT @dbcc_stmt = 'CHECKTABLE'
    DBCC HELP (@dbcc_stmt)
    

    DBCC ind

    Shows all pages in use by indexes of the specified table name.

    Syntax:

    DBCC ind( dbid|dbname, objid, printopt = { 0 | 1 | 2 } )

    where
    
      dbid|dbname - database ID or database name
      objid       - object ID    
      printopt    - print option
                (0  print out only the buffer header and page header (default)
                 1  print page headers, page data in row format, and offset tables
                 2  print page headers, unformatted page data, and offset tables)
    
    Example:

    DBCC TRACEON (3604)
    declare @obid int
    SELECT @obid = object_id('authors')
    DBCC ind (pubs, @obid, 1)
    

    DBCC locateindexpgs

    This command prints all references in the index to the specified page.

    Syntax:

    DBCC locateindexpgs( dbid, objid, page, indexid, level )

    where
    
      dbid    - database ID  
      objid   - object ID  
      page    - logical page number of the page for which index
                references are being searched  
      indexid - index ID  
      level   - level within the index to search for references  
    
    Example:

    DBCC TRACEON (3604)
    DECLARE @dbid int, @objectid int
    SELECT @dbid = DB_ID('pubs')
    SELECT @objectid = object_id('authors')
    DBCC locateindexpgs(@dbid,@objectid,1,1,0)
    

    DBCC lock

    This command can be used to display lock chains.

    Syntax:

    DBCC lock

    Example:

    DBCC TRACEON (3604)
    DBCC lock
    

    DBCC log

    This command is used to view the transaction log for the specified database.

    DBCC log ( {dbid|dbname}, [, type={0|1|2|3|4}] )

    PARAMETERS: 
       Dbid or dbname - Enter either the dbid or the name of the database
                        in question.
    
          type - is the type of output:
    
          0 - minimum information (operation, context, transaction id)
    
          1 - more information (plus flags, tags, row length)
    
          2 - very detailed information (plus object name, index name,
              page id, slot id)
    
          3 - full information about each operation
    
          4 - full information about each operation plus hexadecimal dump
              of the current transaction log's row.
    
    by default type = 0
    
    To view the transaction log for the master database, you can run the following command:

    DBCC log (master)

    DBCC PAGE

    You can use this command to view the data page structure.

    DBCC PAGE ({dbid|dbname}, pagenum [,print option] [,cache] [,logical])

    PARAMETERS: 
       Dbid or dbname - Enter either the dbid or the name of the database
                        in question.
    
       Pagenum - Enter the page number of the SQL Server page that is to
                 be examined.
    
       Print option - (Optional) Print option can be either 0, 1, or 2.
    
                      0 - (Default) This option causes DBCC PAGE to print
                          out only the page header information.
                      1 - This option causes DBCC PAGE to print out the
                          page header information, each row of information
                          from the page, and the page's offset table. Each
                          of the rows printed out will be separated from
                          each other.
                      2 - This option is the same as option 1, except it
                          prints the page rows as a single block of
                          information rather than separating the
                          individual rows. The offset and header will also
                          be displayed.
    
       Cache - (Optional) This parameter allows either a 1 or a 0 to be
               entered.
               0 - This option causes DBCC PAGE to retrieve the page
                   number from disk rather than checking to see if it is
                   in cache.
               1 - (Default) This option takes the page from cache if it
                   is in cache rather than getting it from disk only.
    
       Logical - (Optional) This parameter is for use if the page number
                 that is to be retrieved is a virtual page rather then a
                 logical page. It can be either 0 or 1.
    
                 0 - If the page is to be a virtual page number.
                 1 - (Default) If the page is the logical page number.
    
    In this example, one data page is viewed from the titleauthor table in the pubs database.

    USE pubs
    GO
    DBCC TRACEON (3604)
    GO
    DECLARE @pgid int
    SELECT @pgid = first FROM sysindexes WHERE
      id = object_id('titleauthor') AND indid = 1
    DBCC PAGE (pubs, @pgid, 1)
    GO
    
    This is the result from my computer:

    ...
    
    DATA:
    Offset 32 - 
    011e9820:  04042000 3137322d 33322d31 31373650  .. .172-32-1176P
    011e9830:  53333333 33016400 0000051a 16150f04  S3333.d.........
    ...
    
    Look at here for more details:
    Data page structure in MS SQL 6.5

    DBCC pglinkage

    This command can be used to display the page chain, performing integrity checks during traversal.

    Syntax:

    DBCC pglinkage(dbid,start,number,printopt={0|1|2},target,order={1|0})

    where
    
      dbid     - database ID    
      start    - page number with which to start    
      number   - number of pages to examine, or 0 if target is specified
      printopt - print option
                 (0  display only the count of pages scanned
                  1  display information about the last 16 pages scanned
                  2  display all page numbers in the scan)
      target   - the particular page we are looking for
      order    - traversal order
                 (0  descending, 1  ascending)
    
    Example:

    DBCC TRACEON (3604)
    DBCC pglinkage(6,26,0,1,0,1)
    

    DBCC procbuf

    This command displays procedure buffer headers and stored procedure headers from the procedure cache.

    Syntax:

    DBCC procbuf( [dbid], [objid], [nbufs], [printopt = {0|1}] )

    where
    
      dbid     - database ID    
      objid    - object ID    
      nbufs    - number of buffers to print    
      printopt - print option
                 (0  print out only the proc buff and proc header (default)  
                  1  print out proc buff, proc header and contents of buffer)
    
    Example:

    DBCC TRACEON (3604)
    DECLARE @dbid int, @objectid int
    SELECT @dbid = DB_ID('master')
    SELECT @objectid = object_id('sp_help')
    DBCC procbuf(@dbid,@objectid,1,0)
    

    DBCC prtipage

    This command prints the page number pointed to by each row on the specified index page.

    Syntax:

    DBCC prtipage( dbid, objid, indexid, indexpage )

    where
    
      dbid      - database ID  
      objid     - object ID  
      indexid   - index ID  
      indexpage - the logical page number of the index page to dump
    
    Example:

    DBCC TRACEON (3604)
    DECLARE @dbid int, @objectid int
    SELECT @dbid = DB_ID('pubs')
    SELECT @objectid = object_id('authors')
    DBCC prtipage(@dbid,@objectid,1,0)
    

    DBCC pss

    This command shows info about processes currently connected to the server.

    Syntax:

    DBCC pss( suid, spid, printopt = { 1 | 0 } )

    where
    
      suid     - server user ID     
      spid     - server process ID    
      printopt - print option
                 (0  standard output,
                  1  all open DES's and current sequence tree)
    
    Example:

    DBCC TRACEON (3604)
    dbcc pss
    

    DBCC rebuildextents

    DBCC rebuildextents rebuilds an object's extent chain. You should set the READ ONLY option for your database, before you run this command.

    Syntax:

    DBCC rebuildextents( dbid, objid, indexid )

    where
    
      dbid    - database ID    
      objid   - object ID    
      indexid - index ID    
    
    Example:

    DBCC TRACEON (3604)
    DECLARE @dbid int, @objectid int
    SELECT @dbid = DB_ID('pubs')
    SELECT @objectid = object_id('authors')
    DBCC rebuildextents(@dbid,@objectid,1)
    

    DBCC resource

    This command shows the server's level RESOURCE, PERFMON and DS_CONFIG information. RESOURCE shows addresses of various data structures used by the server. PERFMON structure contains master..spt_monitor field info. DS_CONFIG structure contains master..syscurconfigs field information.

    Syntax:

    DBCC resource

    Example:

    DBCC TRACEON (3604)
    DBCC resource
    

    DBCC show_bucket

    This command shows hash bucket information for the specified pageid.

    Syntax:

    DBCC show_bucket( dbid|dbname, pageid, lookup_type )

    where
    
      dbid|dbname - database ID or database name
      page        - logical page number of page being looked for
      lookup_type - how to conduct search
              (1  use hash algorithm to look in the bucket the page should be in
               2  scan the entire buffer cache)
    
    Example:

    DBCC TRACEON (3604)
    DBCC show_bucket (pubs, 1, 1)
    

    DBCC TAB

    You can use the following undocumented command to view the data pages structure (in comparison with DBCC PAGE, this command will return information about all data pages for viewed table, not only for particular number):

    DBCC tab (dbname, objname, printopt={ 0 | 1 | 2 })

    where
    
          dbname - is the database name,
          objname - is the table name,
          printopt - is the type of the output:
    
          0 - minimum information (only the pages headers, the total
              number of data pages in this table and the total number
              of data rows in this table)
    
          1 - more information (plus full rows structure)
    
          2 - as printopt = 1, but without rows separation (full dump)
    
    by default printopt = 0
    
    Example:

    DBCC TRACEON (3604)
    DBCC tab (pubs, 'authors')
    

    DBCC undo

    Syntax:

    DBCC undo( dbid, pageno, rowno )

    Literature

    1. What are all the dbcc commands for SQL Server?

    2. INFO: Description of DBCC PAGE Command

    3. THE UNAUTHORIZED DOCUMENTATION OF DBCC

    4. The Totally Unauthorized List of Sybase DBCC Commands (1)

    5. The Totally Unauthorized List of Sybase DBCC Commands (2)

    6. The Totally Unauthorized List of Sybase DBCC Commands (3)

    7. FIX: Database Usage Count Does Not Return to Zero

    8. INFO: Description of the DBCC PGLINKAGE Command


     

     
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