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Author: Jugal Shah

  • How to attach a SQL Server database with a missing transaction log file?

    There may be situation where the transaction log file is corrupted or delete due to some issue, there is still hope to recover the database using data file. In the below script we will create the scenario and recover the database using data file only.

    -- create the below sample database
create database [SQLDBPool_5] 

-- execute the command to get the data log file location
sp_helpdb sqlhelpdesk

-- Copy the location of log file and data file from the output of the above command
-- C:\JSpace\sqldbpool_5_log.ldf, C:\JSpace\sqldbpool_5.mdf

-- Detatch the database
USE [master]
GO
ALTER DATABASE [SQLDBPool_5] SET  SINGLE_USER WITH ROLLBACK IMMEDIATE
GO
USE [master]
GO
EXEC master.dbo.sp_detach_db @dbname = N'SQLDBPool_5'
GO

-- Delete the log file
xp_cmdshell 'del "c:\JSpace\SQLDBPool_5_log.ldf"'


-- attach the database using MDF file only SQL Server will create the log file its self
USE [master]
GO
CREATE DATABASE [SQLDBPool_5]  ON 
( FILENAME = N'C:\JSpace\sqldbpool_5.mdf' )
FOR ATTACH
GO

-- You will the below message on the execution of the above command
/*
File activation failure. The physical file name "c:\JSpace\SQLDBPool_5_log.ldf" may be incorrect.
New log file 'c:\JSpace\SQLDBPool_5_log.ldf' was created.
*/

  • LiteSpeed – Restore Script

    Script to check the backup file – data and log file information

    exec master.dbo.xp_restore_filelistonly
@filename ='<BackupFilePath>\<BackupFileName.bak>'

    Script to check the backup file header information

    exec master.dbo.xp_restore_headeronly
@filename ='<BackupFilePath>\<BackupFileName.bak>'

    Script to verify the backup file if the backup file is valid or not

    EXEC master.dbo.xp_restore_verifyonly @filename ='<BackupFilePath>\<BackupFileName.bak>'

    Script to restore database using Full backup with the default options

    exec master.dbo.xp_restore_database
@database = '<dbname>',
@filename = '<BackupFilePath>\<BackupFileName.bak>'

    Script to restore database using Full backup with file move option

    exec master.dbo.xp_restore_database
@database = '<dbname>',
@filename = '<BackupFilePath>\<BackupFileName.bak>',
@with = 'move "logical filename" to "physical file location.mdf"',
@with = 'move "logical filename" to "physical file location.ldf"'

    Script to restore database using Full backup with replace option

    exec master.dbo.xp_restore_database
@database = '<dbname>',
@filename = '<BackupFilePath>\<BackupFileName.bak>',
@with = 'replace',
@with = 'move "logical filename" to "physical file location.mdf"',
@with = 'move "logical filename" to "physical file location.ldf"'

    Script to restore Full backup with no recovery

    exec master.dbo.xp_restore_database
@database = '<dbname>',
@filename = '<BackupFilePath>\<BackupFileName.bak>',
@with = 'replace',
@with = 'move "logical filename" to "physical file location.mdf"',
@with = 'move "logical filename" to "physical file location.ldf"',
@with='NORECOVERY'

    Script to restore log backup with no recovery

    EXEC master.dbo.xp_restore_log
@database = '<dbname>',
@filename = '<BackupFilePath>\<BackupFileName.trn>',
@with ='NORecovery'

    Script to restore log backup with recovery

    EXEC master.dbo.xp_restore_log
@database = '<dbname>',
@filename = '<BackupFilePath>\<BackupFileName.trn>',
@with ='Recovery'

    Script to do point in time recovery

    EXEC master.dbo.xp_restore_log
@database = '<dbname>',
@filename = '<BackupFilePath>\<BackupFileName.trn>',
@with ='Recovery',
@with = 'STOPBEFOREMARK = <LogMark>'

  • LiteSpeed – Backup Scripts

    full or complete database backup

    execute master.dbo.xp_backup_database
  @database = '<database name>',
  @filename = '<path>\<backup file name>.bak',
  @init = 1,
  @compressionlevel = 4

    differential backup

     
 execute master.dbo.xp_backup_database
  @database = '<database name>',
  @filename = '<path>\<backup file name>.bak',
  @init = 1,
  @compressionlevel = 4,
  @with =  differential

    transaction log backup

     
 execute master.dbo.xp_backup_log
  @database = '<database name>',
  @filename = '<path>\<backup file name>.trn',
  @init = 1,
  @compressionlevel = 4

    filegroup backup

       
 execute master.dbo.xp_backup_database 
  @database = '<database name>',
  @filename = '<path>\<backup file name>.bck',
  @init = 1,
  @compressionlevel = 4,
  @filegroup = 'filegroupname'

  • Table Hints – NoLock vs ReadPast

    When any data in a database is read or modified, the database engine uses special type of mechanism, called locks, to maintain integrity in the database. Locks will be used to make sure the transaction consistency.

    NoLock Table Hint
    – Will allow you to read the uncommited data
    – Only used with SELECT statement
    – Blocking will not occur
    – Will reduce the concurrency and improve the performance at some extent
    – Risk of doing Phantom reads

    Let’s create table for the NOLOCK and READPAST hint demo

    create table tranDemo
(
			id int identity(1,1),
			name varchar(10)
)

insert into tranDemo values ('Jugal')
insert into tranDemo values ('Nehal')

    –Now let’s update the values by specifying the explicit transaction and don’t commit/rollback the transaction

    begin transaction
   update tranDemo
   set name = 'DJ'
   where name = 'Jugal'

    Now open new query window and execute the below query and you will notice query will not return data and will continue running as it is blocked

    select * from trandemo

    Now open new query window and execute the below query to check the blocking, you can see the blocking SPID in the result set

    sp_who2 active

    Now open new query window and execute the below query using NOLOCK hint and it will return data, yet transaction is not committed but it will return the updated value.

    select * from trandemo(NOLOCK)

    READPast Table Hint: Less commonly used table hint than NOLOCK. This hint specifies that the database engine not consider any locked rows or data pages when returning results.
    – Will only read the commited rows which are not locked
    – Blocking will not occur
    – Only used with SELECT statement
    – Will reduce the concurrency and improve the performance at some extent
    – Result set returned by this hint is not perfect as it will not retun the locked rows or pages, so you can not make any decision based on data

    Now open new query window and run the below query it will return only one record (“Nehal”) which is not locked or modified

    select * from trandemo(ReadPast)

  • XP_LogEvent and RaiseError

    xp_logevent
    xp_logevent extended stored procedure is used to log the message into the SQL Server error log and event viewer application log.

    Syntax
    xp_logevent { error_number , ‘message’ } [ , ‘severity’ ]
    Parameter Description
    Error number Error number should be greater than 50000
    Message Text you want to record in the error log and event viewer log
    Severity Optional parameter (warning, informational or error) used to determine the type of error that will be logged in the NT application event log.

    Than the question will arise in your mind what is the use of RAISERROR,  here is the difference between XP_LOGEVENT and RAISEERROR

    XP_LOGEVENT RAISEERROR
    Logs a user-defined message in the SQL Server log file and in the Windows Event Viewer. xp_logevent can be used to send an alert without sending a message to the client. Generates an error message. RAISERROR can either reference a user-defined message stored in the sys.messages view or build a message dynamically
    Syntax
    xp_logevent { error_number , ‘message’ } [ , ‘severity’ ]    		 Syntax
    RAISERROR ( { msg_id | msg_str | @local_variable }
    { ,severity ,state }
    [ ,argument [ ,…n ] ] )
    [ WITH option [ ,…n ] ]
    Does not return the error to the client application Does return the error to the client application
    Does not change the value of @@error Does change the value of @@error
    Can only be used to log the messages in the SQL Server error log and event viewer log Can use for error handling and RaiseError with log can perform the similar functionality to XP_LOGEVENT
    Permission
    Requires membership in the db_owner fixed database role in the master database, or membership in the sysadmin fixed server role.    		 Permission
    Severity levels from 0 through 18 can be specified by any user. Severity levels from 19 through 25 can only be specified by members of the sysadmin fixed server role or users with ALTER TRACE permissions

    Example

    EXEC master..xp_logevent 50003, ‘Stored Procedure executed successfully’
    Exec xp_readerrorlog –check the error message in error log or event viewer log