Category Archives: Database

Best Practices SQL Server Transaction Log

Background
In SQL Server Database Recovery model will decide how the transaction log will be logged in transaction log file. Transaction log file extension is .LDF

Full – Transaction log is fully logged (Can take log backup)
Bulk Logged – Bulk transaction is minimally logged (Can take log backup)
Simple – Transaction log will be truncated on checkpoint

In transaction log file transactions are sequentially logged, every record in transaction log file is uniquely identified by log sequence number (LSN). LSN data type is Numeric (25,0)

You can follow below best practices for the transaction log file

1. Don’t create multiple log files : As transactions will be logged into log file sequential manner it would not help for data stripping across multiple files
2. Keep the transaction log file on the separate drive
3. Identify the RPO and RTO for the database and according to that choose the recovery model and correct log backup strategy
4. RAID 1 + 0 is high recommended for transaction log
5. AUTO SHRINK should be always off on the database
6. Pre-allocate the space to transaction log file, it will improve the performance. Don’t depend on the auto growth option.
7. Always set the values of Initial size, max size and growth property of the transaction log file
8. Always set auto growth value, don’t set in percentage
9. Transaction Log file internal fragmentation can also lead the performance and database recovery issue. Database should not have an excessive number of Virtual Log Files (VLFs) inside the Transaction Log. Having a large number of small VLFs can slow down the recovery process that a database goes through on startup or after restoring a backup. Make sure transaction log initial size and log growth defined well to avoid internal fragmentation
10. External fragmentation can be removed by using disk defragmentation utility
11. In case of Transaction log full, please use below query to check the cause of the log full and take the decision accordingly.

SELECT name ,
recovery_model_desc ,
log_reuse_wait_desc
FROM sys.databases
WHERE name = @DatabaseName

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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.
*/

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)

SQL Server 2008: SQL Mail does not work with the 64-bit version of SQL Server (Microsoft SQL Server, Error: 17938)

Recently I got email from one of the blog reader that he is getting below error while setting up SQL MAIL as legacy feature in SQL Server 2008.

Error:
SQL Mail does not work with the 64-bit version of SQL Server (Microsoft SQL Server, Error: 17938)

Solution:
SQL Mail is not supported on 64-bit versions of SQL Server. SQL Mail stored procedures cannot be installed on 64-bit versions. If you want to still use it you have to go with the 32-bit edition or start using database mail, which is more easy.

Follow below article link to start converting your SQLMAIL to DB Mail.
http://msdn.microsoft.com/en-us/library/ms187891.aspx

T-SQL Script to find out the database file size, space used and available free space

While troubleshooting disk space issue, it is essential to know about the database file size statistics. You can execute below script to get database file size information.

set nocount on

create table #dbfileInfo(
name varchar(300),
location varchar(300),
filesizeMB decimal(9,2),
spaceUsedMB decimal(9,2),
FreespaceMB decimal(9,2))

declare @mySQL nvarchar(2000)
DECLARE @dbName varchar(MAX)
DECLARE @cur_DBName CURSOR

SET @cur_DBName = CURSOR FOR
select name from sys.databases

OPEN @cur_DBName
FETCH NEXT
FROM @cur_DBName INTO @dbName
WHILE @@FETCH_STATUS = 0
BEGIN
PRINT @dbName
if DATABASEPROPERTYEX(@dbName, 'status') = 'ONLINE'
begin
select @mySQL = 
    '
        use ' + @dbname + '
        INSERT INTO #dbfileInfo
        select
      name
    , filename
    , convert(decimal(12,2),round(a.size/128.000,2)) as FileSizeMB
    , convert(decimal(12,2),round(fileproperty(a.name,''SpaceUsed'')/128.000,2)) as SpaceUsedMB
    , convert(decimal(12,2),round((a.size-fileproperty(a.name,''SpaceUsed''))/128.000,2)) as FreeSpaceMB
    from dbo.sysfiles a
    '
    exec sp_executesql @mySQL
end
FETCH NEXT
FROM @cur_DBName INTO @dbName

END
CLOSE @cur_DBName
DEALLOCATE @cur_DBName
GO

select * from #dbfileInfo
drop table #dbfileInfo

FileOutput