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 |
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These are 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 |
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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’] |
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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’ |
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