Locking in SQL Server 6.5

,



Introduction

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

SQL Server 6.5 locking, about Lock Escalation, and how you can set

insert row lock for a particular table or tables.

General details

There are three units of data storage in SQL Server 6.5:

  • Page

  • Extent

  • Allocation Unit

SQL Server 6.5 stores data on the 2Kb data pages. Only 1962 bytes

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

to store system information.

See this article for more details:

Data

page structure in MS SQL 6.5

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 is the 8 continuous pages, used by one table. When you create

new table, then new Extent will be generated. One Extent cannot stores

the data from the two or more tables (it's for SQL Server 6.5, not for

SQL Server 7.0).

Allocation Unit contains 32 extents, or 256 pages. It's the largest unit

used by SQL Server 6.5 to store the data.

SQL Server 6.5 can lock the following types of items:

  • Page

  • Extent

  • Table

  • Intent

Page lock is the most common type of lock. When you update or insert

new row, entire page will be locked. SQL Server automatically

generates page-level locks, when a query requests rows from a table.

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. You can set this percentage with sp_configure

system stored procedure to the appropriate value. This is described

below in the Lock Escalation item.

Intent lock is a special type of table lock to indicate the type of

page locks currently placed on the table.

There is also insert row locking. The insert row locking can be set with

sp_tableoption system stored procedure and will be described below.

Lock Escalation

You can customize locking by setting Lock Escalation level. The Lock

Escalation level determines, when SQL Server applies table locks instead

of page locks, and it affects all users of SQL Server. So it's escalation

from the page's to the table's level locking.

There are three Lock Escalation options:

  • LE threshold maximum

  • LE threshold minimum

  • LE threshold percent

LE threshold maximum is the maximum number of page locks to hold

before escalating to a table lock. The default value is 200.

LE threshold minimum is the minimum number of page locks required

before escalating to a table lock. The default value is 20.

LE threshold percent is the percentage of page locks needed on a

table before escalating to a table lock. The default value is 0, it means

that a table lock will be occur only when the LE threshold maximum will

be exceeded.

You can configure Lock Escalation levels by using the sp_configure

system stored procedure.

This is the example to set LE threshold maximum to 250:

EXEC sp_configure 'LE threshold maximum'

GO

EXEC sp_configure 'LE threshold maximum', 250

GO

RECONFIGURE WITH OVERRIDE

GO

EXEC sp_configure 'LE threshold maximum'

GO

This is the results:

name                                minimum     maximum     config_value

run_value

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

-----------

LE threshold maximum 2 500000 200 200

Configuration option changed. Run the RECONFIGURE command to install.

name minimum maximum config_value

run_value

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

-----------

LE threshold maximum 2 500000 250 250

Setting insert row lock

You can set insert row lock option for the user-defined table/tables

with sp_tableoption system stored procedure.

This is the syntax from SQL Server Books Online:

sp_tableoption @TableNamePattern  [, '@OptionName'] [, '@OptionValue']

where

@TableNamePattern
is a user-defined database table.
@OptionName
is a option name.

There are insert row lock and pintable options.

@OptionValue is a option value. @OptionValue can be 'false' or 'true'.

The default value for insert row lock option is 'false'.

This is the example to set insert row lock option for authors table:

EXEC sp_tableoption 'authors', 'insert row lock', 'true'

Rate

Share

Share

Rate