Deadlocks in SQL Server 2008

There are few different kinds of lock available in SQL Server but the most resource consuming especially CPU power is DeadLock. It is better to know about deadlock and try to reduce as much as possible. First of all let us understand what is deadlock and how deadlock happens?

Deadlock happens when two or more processes waiting for the resource and wait for other process to finish and eventually neither of them ever does it at that time SQL Server select one process and abort other process.

Let us see one small example to create dead lock.

--create one two tables for demo and insert dummy records.

create table DL1

(

id int,

name varchar(10)

)

go

create table DL2

(

id int,

name varchar(10)

)

go

insert into DL1

select 1,'ritesh' union all

select 2,'rajan'

go

insert into DL2

select 1,'vipul' union all

select 2,'darshan'

go

--open other query window, will consider this window as A1

begin tran

update dl1 set name='rit' where id=1

go

--open other query window, will consider this window as A2

begin tran

update dl2 set name='vi' where id=2

go

--again come back to window A1

update dl2 set name='d' where id=2

go

Now this statement will create lock and query got hold but it is not a dead lock so far.

--now comeback to A2 window and execute following command

update dl1 set name='raj' where id=1

go

Now, this statement creates dead lock and you will be greeted with the error message something like below and your

Msg 1205, Level 13, State 45, Line 2

Transaction (Process ID 56) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction.

As soon as you execute COMMIT or ROLLBACK, your lock gets release. So, this is how deadlock generates so while designing the database, keep relatively normalize. Keep your transaction as short as possible and try to use TRY….CATCH block so if there is any error in your TRY block, it transfer the focuses to CATCH block and you can ROLLBACK transaction from there.

BTW, if you want to find tables which are being locked, have a look at my article here.

Reference: Ritesh Shah
http://www.sqlhub.com
Note: Microsoft Books online is a default reference of all articles but examples and explanations prepared by Ritesh Shah, founder of http://www.SQLHub.com