- General Concepts
- Declaring a Cursor
- SQL-92 Syntax
- Transact-SQL Extended Syntax
- Opening a Cursor
- Fetching a Cursor
- Closing a Cursor
- Deallocating a Cursor
- Cursor Optimization Tips
- Literature
General concepts
In this article, I want to tell you how to create and use server
side cursors and how you can optimize a cursor performance.
Cursor is a database object used by applications to manipulate data
in a set on a row-by-row basis, instead of the typical SQL commands
that operate on all the rows in the set at one time. For example,
you can use cursor to include a list of all user databases and make
multiple operations against each database by passing each database
name as a variable.
The server side cursors were first added in SQL Server 6.0 release and
now supported in all editions of SQL Server 7.0 and SQL Server 2000.
Before using cursor, you first must declare the cursor. Once a cursor
has been declared, you can open it and fetch from it. You can fetch
row by row and make multiple operations on the currently active row
in the cursor. When you have finished working with a cursor, you
should close cursor and deallocate it to release SQL Server resources.
Declaring a Cursor
Before using cursor, you first must declare the cursor, i.e. define
its scrolling behavior and the query used to build the result set on
which the cursor operates. To declare cursor, you can use a syntax
based on the SQL-92 standard and a syntax using a set of Transact-SQL extensions.
SQL-92 Syntax
This is SQL-92 Syntax:
|
where
cursor_name - the name of the server side cursor, must contain
from 1 to 128 characters.
INSENSITIVE - specifies that cursor will use a temporary copy of the data
instead of base tables. This cursor does not allow modifications
and modifications made to base tables are not reflected in the
data returned by fetches made to this cursor.
SCROLL - specifies that cursor can fetch data in all directions, not only
sequentially until the end of the result set. If this argument is
not specified, FETCH NEXT is the only fetch option supported.
select_statement - the standard select statement, cannot contain
COMPUTE, COMPUTE BY, FOR BROWSE, and INTO keywords.
READ ONLY - specifies that cursor cannot be updated.
UPDATE [OF column_name [,...n]] - specifies that all cursor's columns can be
updated (if OF column_name [,...n] is not
specified), or only the columns listed in the
OF column_name [,...n] list allow modifications.
Cursor Options Compatibility
| INSENSITIVE | SCROLL | READ ONLY | UPDATE | |
| INSENSITIVE | Yes | Yes | No | |
| SCROLL | Yes | Yes | Yes | |
| READ ONLY | Yes | Yes | No | |
| UPDATE | No | Yes | No |
Transact-SQL Extended Syntax
This is Transact-SQL Extended Syntax:
|
where
cursor_name - the name of the server side cursor, must contain
from 1 to 128 characters.
LOCAL - specifies that cursor can be available only in the batch,
stored procedure, or trigger in which the cursor was created.
The LOCAL cursor will be implicitly deallocated when the batch,
stored procedure, or trigger terminates.
GLOBAL - specifies that cursor is global to the connection. The GLOBAL
cursor will be implicitly deallocated at disconnect.
FORWARD_ONLY - specifies that cursor can only fetch data sequentially
from the first to the last row. FETCH NEXT is the only
fetch option supported.
STATIC - specifies that cursor will use a temporary copy of the data
instead of base tables. This cursor does not allow modifications
and modifications made to base tables are not reflected in the
data returned by fetches made to this cursor.
KEYSET - specifies that cursor uses the set of keys that uniquely
identify the cursor's rows (keyset), so that the membership and
order of rows in the cursor are fixed when the cursor is opened.
SQL Server uses a table in tempdb to store keyset. The KEYSET
cursor allows updates nonkey values from being made through
this cursor, but inserts made by other users are not visible.
Updates nonkey values made by other users are visible as the
owner scrolls around the cursor, but updates key values made
by other users are not visible. If a row is deleted, an attempt
to fetch the row returns an @@FETCH_STATUS of -2.
DYNAMIC - specifies that cursor reflects all data changes made to the
base tables as you scroll around the cursor. FETCH ABSOLUTE
option is not supported with DYNAMIC cursor.
FAST_FORWARD - specifies that cursor will be FORWARD_ONLY and READ_ONLY
cursor. The FAST_FORWARD cursors produce the least amount
of overhead on SQL Server.
READ ONLY - specifies that cursor cannot be updated.
SCROLL_LOCKS - specifies that cursor will lock the rows as they are
read into the cursor to ensure that positioned updates
or deletes made through the cursor will be succeed.
OPTIMISTIC - specifies that cursor does not lock rows as they are
read into the cursor. So, the positioned updates or
deletes made through the cursor will not succeed if
the row has been updated outside the cursor since
this row was read into the cursor.
TYPE_WARNING - specifies that if the cursor will be implicitly
converted from the requested type to another,
a warning message will be sent to the client.
select_statement - the standard select statement, cannot contain
COMPUTE, COMPUTE BY, FOR BROWSE, and INTO keywords.
UPDATE [OF column_name [,...n]] - specifies that all cursor's columns can be
updated (if OF column_name [,...n] is not
specified), or only the columns listed in the
OF column_name [,...n] list allow modifications.
Opening a Cursor
Once a cursor has been declared, you must open it to fetch data from it.
To open a cursor, you can use the following syntax:
|
where
GLOBAL - if this argument was not specified and both a global and
a local cursor exist with the same name, the local cursor
will be opened; otherwise, the global cursor will be opened.
cursor_name - the name of the server side cursor, must contain
from 1 to 128 characters.
cursor_variable_name - the name of a cursor variable that
references a cursor.
After a cursor is opening, you can determine the number of rows
that were found by the cursor. To get this number, you can use
@@CURSOR_ROWS scalar function.
Fetching a Cursor
Once a cursor has been opened, you can fetch from it row by row and make
multiple operations on the currently active row in the cursor.
To fetch from a cursor, you can use the following syntax:
|
where
NEXT - the default cursor fetch option. FETCH NEXT returns the
next row after the current row.
PRIOR - returns the prior row before the current row.
FIRST - returns the first row in the cursor.
LAST - returns the last row in the cursor.
ABSOLUTE {n @nvar} - returns the nth row in the cursor. If a positive
number was specified, the rows are counted from the
top of the data set; if 0 was specified, no rows are
returned; if a negative number was specified, the
number of rows will be counted from the bottom of
the data set.
RELATIVE {n @nvar} - returns the nth row in the cursor relative to
the current row. If a positive number was specified,
returns the nth row beyond the current row; if a
negative number was specified, returns the nth row
prior the current row; if 0 was specified, returns
the current row.
GLOBAL - if this argument was not specified and both a global and
a local cursor exist with the same name, the local cursor
will be fetched; otherwise, the global cursor will be fetched.
cursor_name - the name of the server side cursor, must contain
from 1 to 128 characters.
cursor_variable_name - the name of a cursor variable that
references a cursor.
INTO @variable_name[,...n] - allows data returned from the cursor
to be held in temporary variables. The type
of variables must match the type of columns
in the cursor select list or support implicit
conversion. The number of variables must match
the number of columns in the cursor select list.
Closing a Cursor
When you have finished working with a cursor, you can close it to
release any resources and locks that SQL Server may have used while
the cursor was open.
To close a cursor, you can use the following syntax:
|
where
GLOBAL - if this argument was not specified and both a global and
a local cursor exist with the same name, the local cursor
will be closed; otherwise, the global cursor will be closed.
cursor_name - the name of the server side cursor, must contain
from 1 to 128 characters.
cursor_variable_name - the name of a cursor variable that
references a cursor.
Note. If you have closed a cursor, but have not deallocated it,
you can open it again when needed.
Deallocating a Cursor
When you have finished working with a cursor and want to completely
release SQL Server resources that were used by a cursor, you can
deallocate a cursor.
To deallocate a cursor, you can use the following syntax:
|
where
GLOBAL - if this argument was not specified and both a global and
a local cursor exist with the same name, the local cursor will be
deallocated; otherwise, the global cursor will be deallocated.
cursor_name - the name of the server side cursor, must contain
from 1 to 128 characters.
cursor_variable_name - the name of a cursor variable that
references a cursor.
Note. Deallocating a cursor completely removes all cursor references.
So, after a cursor is deallocated, it no longer can be opened.
Cursor Optimization Tips
- Try to avoid using SQL Server cursors, whenever possible.
SQL Server cursors can results in some performance degradation in comparison
with select statements. Try to use correlated subquery or derived tables, if
you need to perform row-by-row operations. - Do not forget to close SQL Server cursor when its result set is not needed.
To close SQL Server cursor, you can use CLOSE {cursor_name} command. This command
releases the cursor result set and frees any cursor locks held on the rows on
which the cursor is positioned. - Do not forget to deallocate SQL Server cursor when the data structures
comprising the cursor are not needed.
To deallocate SQL Server cursor, you can use DEALLOCATE {cursor_name} command.
This command removes a cursor reference and releases the data structures comprising
the cursor. - Try to reduce the number of records to process in the cursor.
To reduce the cursor result set, use the WHERE clause in the cursor's
select statement. It can increase cursor performance and reduce SQL Server
overhead. - Try to reduce the number of columns to process in the cursor.
Include in the cursor's select statement only necessary columns. It will
reduce the cursor result set. So, the cursor will use fewer resources.
It can increase cursor performance and reduce SQL Server overhead. - Use READ ONLY cursors, whenever possible, instead of updatable cursors.
Because using cursors can reduce concurrency and lead to unnecessary locking,
try to use READ ONLY cursors, if you do not need to update cursor result set. - Try avoid using insensitive, static and keyset cursors, whenever possible.
These types of cursor produce the largest amount of overhead on SQL Server,
because they cause a temporary table to be created in TEMPDB, which results
in some performance degradation. - Use FAST_FORWARD cursors, whenever possible.
The FAST_FORWARD cursors produce the least amount of overhead on SQL Server,
because there are read-only cursors and can only be scrolled from the first
to the last row. Use FAST_FORWARD cursor if you do not need to update cursor
result set and the FETCH NEXT will be the only used fetch option. - Use FORWARD_ONLY cursors, if you need updatable cursor and the FETCH NEXT
will be the only used fetch option.
If you need read-only cursor and the FETCH NEXT will be the only used fetch
option, try to use FAST_FORWARD cursor instead of FORWARD_ONLY cursor.
By the way, if one of the FAST_FORWARD or FORWARD_ONLY is specified the
other cannot be specified.
Literature
1. DECLARE CURSOR
2. OPEN
3. FETCH
4. http://msdn.microsoft.com/library/en-us/tsqlref/ts_globals_1c1f.asp
5. CLOSE
6. DEALLOCATE
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