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LOCK in SQL Server




In SQL Server 2000 (Enterprise Manager)

1. Expand server – management-current Activity-expand
Locks/processid and you will be able to see all the locks related information.

2. Expand server – management-current Activity-expand Locks/object you can see locks by object information.

In SQL Server 2005 (SSMS, object Explorer)

Expand-server-management-double click Activity Monitor.

On left side you have three options to choose from, select those options and you can see all the locks related information.

Run this stored procedure in the database.

1. sp_lock

To know the running process in the sql server, run this query,

2. select * from sysprocesses (in sql server 2000)
3. select * from sys.sysprocesses (in sql server 2005)

4. sp_who
5. sp_who2 will also give you some good information.

To work around the locks, you can run profiler to check which query is creating a lock and if that is necessary.

Types of locks on object level, (general idea)

Database: Database.
Extent: Contiguous group of eight data pages or index pages.
Key: Row lock within an index.
Page: 8-kilobyte (KB) data page or index page.
RID: Row ID, Used to lock a single row within a table.
Table: Entire table, including all data and indexes.

Types of locks:
Shared (S) – more than one Query can access the object.
Exclusive lock (X) – only one Query can access the object.
Update lock (U)
Intent share (IS)
Intent Exclusive (IX)

Just to give you a brief idea about locks, we have something called as transaction levels in sql server databases.

TRANSACTION ISOLATION LEVEL

Level 0: READ COMMITTED
Level 1: READ UNCOMMITTED
Level 2: REPEATABLE READ
Level 3: SERIALIZABLE

Level 0 is the lowest level isolation level, if your database is set in this isolation level, no query will lock any resources, under this level, and there will be no locks on the database, not even shared locks.

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED

This data will also read uncommitted data. Data which you have not committed, you can still read that data.

Level1 is the default isolation level of the database.
Under this category you will not be able to read uncommitted data; this is also called as dirty data. Under this we will have shared locks.

As the level increases the locks also increases. The highest is the serializable.

To make you understand in detail, let’s see an example of what is committed data and what is uncommitted data.

use pubs
create table example1 ( eid int, ename varchar(10))

begin tran T1
insert into example1 values ( 1, ‘example’)
go

select * from example1 — this is uncommitted data.

The above is uncommitted transaction, because you started the transaction with a begin, you have to commit the transaction, until then the transaction will not be uncommitted.

To commit the same transaction


commit tran T1

select * from example1 — this is committed data.

To check the current isolation level of your database, run this command,

Dbcc useroptions — check for isolation level.

If you don’t want your query to put locks on objects you might want to use something like this,

select * from example1_1 with (nolock)

This will not keep any lock, not even a shared lock on the table. This is the most effective way to avoid locks in SQL.

This is in-depth concept. Hope this will help someone.


Microsoft SQL Server 2005 UnleashedExpert SQL Server 2005 DevelopmentMicrosoft SQL Server 2005 For DummiesMaster Lock 178D Set-Your-Own Combination Padlock, Die-Cast, Black

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