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MS SQL

Posted May 20, 2002

Using SQL Server Cursors

By Alexander Chigrik


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 the SQL Server 6.0 release and are 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:

    DECLARE cursor_name [INSENSITIVE] [SCROLL] CURSOR
    FOR select_statement
    [FOR {READ ONLY | UPDATE [OF column_name [,...n]]}]
    

    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:

    DECLARE cursor_name CURSOR
    [LOCAL | GLOBAL]
    [FORWARD_ONLY | SCROLL]
    [STATIC | KEYSET | DYNAMIC | FAST_FORWARD]
    [READ_ONLY | SCROLL_LOCKS | OPTIMISTIC]
    [TYPE_WARNING]
    FOR select_statement
    [FOR UPDATE [OF column_name [,...n]]]
    

    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.

    Cursor Options Compatibility

    LOCAL GLOBAL FORWARD
    ONLY
    STATIC KEYSET DYNAMIC FAST
    FORWARD
    READ
    ONLY
    SCROLL
    LOCKS
    OPTIMISTIC TYPE
    WARNING
    UPDATE
    LOCAL No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
    GLOBAL No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
    FORWARD_ONLY Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes
    STATIC Yes Yes Yes No No No Yes No Yes Yes No
    KEYSET Yes Yes Yes No No No Yes Yes Yes Yes Yes
    DYNAMIC Yes Yes Yes No No No Yes Yes Yes Yes Yes
    FAST_FORWARD Yes Yes No No No No Yes No No Yes No
    READ_ONLY Yes Yes Yes Yes Yes Yes Yes No No Yes No
    SCROLL_LOCKS Yes Yes Yes No Yes Yes No No No Yes Yes
    OPTIMISTIC Yes Yes Yes Yes Yes Yes No No No Yes Yes
    TYPE_WARNING Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
    UPDATE Yes Yes Yes No Yes Yes No No Yes Yes Yes


    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:

    OPEN { { [GLOBAL] cursor_name } | cursor_variable_name}
    

    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:

    FETCH
            [    [    NEXT | PRIOR | FIRST | LAST
                    | ABSOLUTE {n | @nvar}
                    | RELATIVE {n | @nvar}
                ]
                FROM
            ]
    { { [GLOBAL] cursor_name } | @cursor_variable_name}
    [INTO @variable_name[,...n] ]
    

    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:

    CLOSE { { [GLOBAL] cursor_name } | cursor_variable_name }
    

    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:

    DEALLOCATE { { [GLOBAL] cursor_name } | @cursor_variable_name}
    

    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.

      Using SQL Server cursors can result 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 the 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 the 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. This 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 as 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 as they 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. Chapter 15 - Creating and Using Cursors
    Special Edition Using SQL Server
    by Bob Branchek, Peter Hazlehurst, Stephen Wynkoop, Scott L. Warner

    2. Performance Tuning SQL Server Cursors

    3. MSDN Library - DECLARE CURSOR

    4. MSDN Library - OPEN

    5. MSDN Library - FETCH

    6. MSDN Library - @@FETCH_STATUS

    7. MSDN Library - CLOSE

    8. MSDN Library - DEALLOCATE


    » See All Articles by Columnist Alexander Chigrik




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