SQLServerCentral Article

Locking in SQL Server 7.0

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Introduction

In this article, I want to tell you about general details of

SQL Server 7.0 locking, about Transaction Isolation Levels,

what kind of Transaction Isolation Level exists, and how you

can set the appropriate Transaction Isolation Level, about

Lock types and Locking optimizer hints, about Lock Escalation,

about Deadlocks, and about how you can view locks with sp_lock

or sp_lock2 stored procedure.

General details

There are two units of data storage in SQL Server 7.0:

  • Page

  • Extent

SQL Server 7.0 stores data on the 8Kb data pages. Only 8060 bytes are

used to store the user's data, other space is used by SQL Server to

store system information.

When you insert a new row, and there is no space on the current data

page to store this row, then new page will be created.

Extent consists of eight 8 KB pages. When you create new table, then

new Extent will be generated. In SQL Server 7.0, different objects can

share an extent or an object can have its own extent(s). A table and

index both have a minimum of two pages.

SQL Server 7.0 can lock the following types of items:

  • RID

  • Key

  • Page

  • Extent

  • Table

  • Database

RID is a row identifier. It is used to individually lock a single row

within a table.

Key is a row lock within an index. Used to protect key ranges in

serializable transactions.

Page lock is a lock, when entire 8-KB data page or index page will

be locked.

Extent lock is only used for allocation. When it's used, entire

extent

will be locked.

Table lock is used when a large percentage of the table's rows are

queried or updated. This lock includes all table's data and indexes.

Database lock is used when you restore the database.

Transaction Isolation Levels

There are four isolation levels:

  • READ UNCOMMITTED

  • READ COMMITTED

  • REPEATABLE READ

  • SERIALIZABLE

SQL Server 7.0 supports all of these Transaction Isolation Levels and can

separate REPEATABLE READ and SERIALIZABLE.

Let me to describe each isolation level.

READ UNCOMMITTED

When it's used, SQL Server not issue shared locks while reading data. So,

you

can read an uncommitted transaction that might get rolled back later. This

isolation level is also called dirty read. This is the lowest isolation

level.

It ensures only that a physically corrupt data will not be read.

READ COMMITTED

This is the default isolation level in SQL Server. When it's used, SQL

Server

will use shared locks while reading data. It ensures that a physically

corrupt

data will not be read and will never read data that another application has

changed and not yet committed, but it not ensures that the data will not be

changed before the end of the transaction.

REPEATABLE READ

When it's used, then dirty reads and nonrepeatable reads cannot occur.

It means that locks will be placed on all data that is used in a query,

and another transactions cannot update the data.

This is the definition of nonrepeatable read from SQL Server Books Online:

nonrepeatable read

When a transaction reads the same row more than one time, and between the

two (or more) reads, a separate transaction modifies that row. Because the

row was modified between reads within the same transaction, each read

produces different values, which introduces inconsistency.

SERIALIZABLE

Most restrictive isolation level. When it's used, then phantom values cannot

occur. It prevents other users from updating or inserting rows into the data

set until the transaction is complete.

This is the definition of phantom from SQL Server Books Online:

phantom

Phantom behavior occurs when a transaction attempts to select a row that

does not exist and a second transaction inserts the row before the first

transaction finishes. If the row is inserted, the row appears as a phantom

to the first transaction, inconsistently appearing and disappearing.

You can set the appropriate isolation level for an entire SQL Server session

with the SET TRANSACTION ISOLATION LEVEL statement.

This is the syntax from SQL Server Books Online:

SET TRANSACTION ISOLATION LEVEL

{

READ COMMITTED

| READ UNCOMMITTED

| REPEATABLE READ

| SERIALIZABLE

}

You can use DBCC USEROPTIONS command to determine the Transaction Isolation

Level currently set. This command returns the set options that are active

for the current connection. This is the example:

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED

GO

DBCC USEROPTIONS

GO

Lock types

There are three main types of locks that SQL Server 7.0 uses:

  • Shared locks

  • Update locks

  • Exclusive locks

Shared locks are used for operations that do not change or update

data, such as a SELECT statement.

Update locks are used when SQL Server intends to modify a

page, and later promotes the update page lock to an exclusive page

lock before actually making the changes.

Exclusive locks are used for the data modification operations,

such as UPDATE, INSERT, or DELETE.

Shared locks are compatible with other Shared locks or

Update locks.

Update locks are compatible with Shared locks only.

Exclusive locks are not compatible with other lock types.

Let me to describe it on the real example. There are four processes,

which attempt to lock the same page of the same table. These processes

start one after the other, so Process1 is the first process, Process2

is the second process and so on.

Process1 : SELECT
Process2 : SELECT
Process3 : UPDATE
Process4 : SELECT

Process1 sets the Shared lock on the page, because there are

no another locks on this page.

Process2 sets the Shared lock on the page, because Shared

locks are compatible with other Shared locks.

Process3 wants to modify data and wants to set Exclusive lock, but it

cannot make it before Process1 and Process2 will be finished, because

Exclusive lock is not compatible with other lock types. So,

Process3 sets Update lock.

Process4 cannot set Shared lock on the page before Process3 will be

finished. So, there are no Lock starvation. Lock starvation

occurs when read transactions can monopolize a table or page, forcing

a write transaction to wait indefinitely. So, Process4 waits before

Process3 will be finished.

After Process1 and Process2 were finished, Process3 transfer Update

lock into Exclusive lock to modify data. After Process3 was finished,

Process4 sets the Shared lock on the page to select data.

Locking optimizer hints

There are twelve Locking optimizer hints in SQL Server 7.0:

  • NOLOCK

  • HOLDLOCK

  • UPDLOCK

  • TABLOCK

  • PAGLOCK

  • TABLOCKX

  • READCOMMITTED

  • READUNCOMMITTED

  • REPEATABLEREAD

  • SERIALIZABLE

  • READPAST

  • ROWLOCK

The first six Locking optimizer hints are used in SQL Server 6.5 and

was described in my previous article about "Locking in SQL Server 6.5".

See this article for more information:

Locking in

SQL Server 6.5

The last six Locking optimizer hints are the new SQL Server 7.0 hints.

Let me to describe them.

NOLOCK is also known as "dirty reads". This option directs SQL Server

not to issue shared locks and not to honor exclusive locks. So, if this

option is specified, it is possible to read an uncommitted transaction.

This results in higher concurrency and in lower consistency.

HOLDLOCK directs SQL Server to hold a shared lock until completion

of the transaction in which HOLDLOCK is used. You cannot use HOLDLOCK

in a SELECT statement that includes the FOR BROWSE option. HOLDLOCK is

equivalent to SERIALIZABLE.

UPDLOCK instructs SQL Server to use update locks instead of shared

locks while reading a table and holds them until the end of the command

or transaction.

TABLOCK takes a shared lock on the table that is held until the

end of the command. if you also specify HOLDLOCK, the lock is held until

the end of the transaction.

PAGLOCK is used by default. Directs SQL Server to use shared page

locks.

TABLOCKX takes an exclusive lock on the table that is held until the

end of the command or transaction.

READCOMMITTED performs a scan with the same locking semantics as a

transaction running at the READ COMMITTED isolation level. By default,

SQL Server operates at this isolation level.

READUNCOMMITTED is equivalent to NOLOCK.

REPEATABLEREAD performs a scan with the same locking semantics as a

transaction running at the REPEATABLE READ isolation level.

SERIALIZABLE performs a scan with the same locking semantics as a

transaction running at the SERIALIZABLE isolation level. Equivalent to

HOLDLOCK.

READPAST skips locked rows. This option causes a transaction to skip

over rows locked by other transactions that would ordinarily appear in

the result set. The READPAST lock hint applies only to the SELECT statement.

Note You can only specify the READPAST lock in the READ COMMITTED

or REPEATABLE READ isolation levels.

ROWLOCK uses row-level locks rather than use the page or table-level

locks.

You can specify one of these locking options in a SELECT statement.

This is the example:

SELECT au_fname FROM pubs..authors (holdlock)

Lock Escalation

This is the description of the Lock Escalation process from the

SQL Server Books Online:

Lock escalation is the process of converting many fine-grain locks

into fewer coarse-grain locks, reducing system overhead.

You cannot customize locking by setting Lock Escalation level in

SQL Server 7.0. There is no such option now. SQL Server 7.0

automatically escalates row locks and page locks into table locks

when a transaction exceeds its escalation threshold.

Lock escalation thresholds are determined dynamically by SQL Server 7.0

and cannot be configured manually.

Deadlocks

Deadlock occurs when two users have locks on separate objects and

each user wants a lock on the other's object. For example, User1

has a lock on object "A" and wants a lock on object "B", and User2

has a lock on object "B" and wants a lock on object "A". In this

case, SQL Server 7.0 ends a deadlock by choosing the user, who will

be a deadlock victim. After that, SQL Server rolls back the breaking

user's transaction, sends message number 1205 to notify the user's

application about breaking, and then allows the nonbreaking user's

process to continue.

You can decide which connection will be the candidate for deadlock

victim by using SET DEADLOCK_PRIORITY. In other case, SQL Server

selects the deadlock victim by choosing the process that completes

the circular chain of locks.

So, in a multiuser situation, your application should check for

message 1205 to indicate that the transaction was rolled back and

if so, to restart the transaction.

Note To reduce the chance of a deadlock, you should minimize the

size of transactions and transaction times.

Viewing locks (sp_lock)

Sometimes you need a reference to information about locks.

Microsoft recommends to use sp_lock system stored procedure

to report locks information. This very useful procedure returns the

information about SQL Server process ID, which lock the data, about

locked database ID, about locked object ID, about locked index ID

and about type of locking (type, resource, mode and status columns).

This is the results set of sp_lock stored procedure:

spid   dbid   ObjId       IndId  Type Resource         Mode     Status

------ ------ ----------- ------ ---- ---------------- -------- ------

1 1 0 0 DB S GRANT

6 1 0 0 DB S GRANT

7 1 0 0 DB S GRANT

8 1 0 0 DB S GRANT

9 1 0 0 DB S GRANT

9 2 0 0 DB S GRANT

9 1 117575457 0 TAB IS GRANT

10 1 0 0 DB S GRANT

11 5 0 0 DB S GRANT

The information, returned by sp_lock stored procedure, is needed in

some clarification, because it's difficult to understand database

name, object name and index name by their ID numbers.

Microsoft provides an enhanced version of the sp_lock system stored

procedure, which returns user name, host name, database name and

object name also.

You can find this stored procedure at here:

INF: sp_lock2 Returns Additional Locking Details

http://support.microsoft.com/support/kb/articles/Q255/5/96.ASP

This enhanced stored procedure works under SQL Server 7.0 as well, but

has syntax error under SQL Server 2000. It does not return the name of

an index also.

Here you can find the new version of the sp_lock2 stored procedure

for SQL Server 7.0 and SQL Server 2000. This version returns index

name and object owner also:

Detailed

locking view: sp_lock2

Literature

  1. SQL Server Books Online
  2. Transaction Isolation Level
  3. INF: sp_lock2 Returns Additional Locking Details

    http://support.microsoft.com/support/kb/articles/Q255/5/96.ASP

  4. Detailed locking view: sp_lock2
  5. INF: Analyzing and Avoiding Deadlocks in SQL Server

    http://support.microsoft.com/support/kb/articles/Q169/9/60.ASP

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