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Using Cursor Types

A cursor's type, in ADO terminology, indicates some facts about how it behaves, what you can and can't do with it, and how thrifty or wasteful it is with system resources.

You can set a Recordset's cursor type by setting its CursorType just before you open it, as shown in Listing 9.19

LISTING 9.19
SETTING A Recordset'S CURSORTYPE PROPERTY

rsEmployees.CursorType = adOpenStatic
rsEmployees.Open

NOTE - The Most Economical Cursor: The most efficient cursor in terms of resource usage is a Forward-Only cursor with its lock type set to Read- Only. This type of cursor is also known as a "firehose cursor." See the section titled "Using Locking Strategies to Ensure Data Integrity" for more information on the Read-Only lock type.

NOTE - Static Cursors and Client-Side Cursors: When you set the CursorLocation Property to Client-Side, the only available CursorType is Static

The following sections discuss the four cursor types available from the ADO Cursor library.

  1. Forward-Only Cursors

  2. Static Cursors

  3. Keyset Cursors

  4. Dynamic Cursors

Forward-Only Cursors

A Forward-Only cursor behaves a lot like sequential file access: It only furnishes one record at a time, and then only in strict order from the beginning to the end of the rowset.

In other words, you can't use a Forward-Only cursor to skip around in a Recordset's rows. You can only move forward one record at a time until you reach the EOF condition at the end of the Recordset.

If you attempt to use any other Recordset navigation method besides MoveNext, you will generate a runtime error.

A Forward-Only cursor is the default ADO cursor, because it consumes the least resources of all cursor types.

Static Cursors

Static cursors are less economical than Forward-Only cursors, but they allow greater flexibility of movement through the rowset. A Static cursor supports navigation in all directions, and it enables you to make repeat visits to the same record during the same session.

The biggest drawback to a Static cursor is the fact that its rowset doesn't get updated with concurrent changes made by other users.

If User A opens a Static cursor on a set of records and User B makes changes to the records during User A's session, for example, User A will not see the changes made by User B. To see User B's changes, User A's Static cursor would have to close and then be reopened.

The user can make changes to the Static cursor's Recordset, but (once again) the user cannot see changes made by others during the time that the cursor is open. This includes additions and deletions to the records as well as editing changes to individual records.

Keyset Cursors

Keyset-type cursors have the same freedom of movement in any direction as Static cursors. In addition, Keyset cursors can immediately see changes to existing records made by other users. However, additions and deletions made by other users are not visible to a Keyset-type cursor.

Dynamic Cursors

Dynamic cursors have all the flexibility and visibility of Keyset-type cursors with an extra enhancement: Additions and deletions made by other users are visible to a Dynamic cursor. Dynamic cursors are, however, the biggest resource hogs, so you should be very sure that you absolutely need a Dynamic cursor before deciding to use one.


  

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