Keyset-driven Cursors (Database Engine)
The membership and order of rows in a keyset-driven cursor are fixed when the cursor is opened. Keyset-driven cursors are controlled by a set of unique identifiers, keys, known as the keyset. The keys are built from a set of columns that uniquely identify the rows in the result set. The keyset is the set of the key values from all the rows that qualified for the SELECT statement at the time the cursor was opened. The keyset for a keyset-driven cursor is built in tempdb when the cursor is opened.
Changes to data values in nonkeyset columns (made by the cursor owner or committed by other users) are visible as the user scrolls through the cursor. Inserts to the database made outside the cursor are not visible in the cursor unless the cursor is closed and reopened. Inserts made through the cursor using an API function such as the ODBC SQLSetPos function are visible at the end of the cursor. @@FETCH_STATUS returns a "row missing" status when an attempt is made to fetch a row deleted after the cursor was opened. An update to a key column operates like a delete of the old key value followed by an insert of the new key value. The new key value is not visible if the update was not made through the cursor; it is visible at the end of the cursor if the update was made through the cursor using either an API function such as SQLSetPos or the Transact-SQL WHERE CURRENT OF clause and the SELECT statement did not contain a JOIN condition in the FROM clause. The new key value is not visible if the insert contained a remote table in the FROM clause. Attempts to retrieve the old key value get the same missing row fetch status as a deleted row.
|An index on computed columns cannot be used when declaring a keyset-driven cursor on a remote table. You can create another index to provide the unique keys for that remote table.|