An update cursor is a special kind of cursor that applications
can use when the row might potentially be updated. To use an update
cursor, execute SELECT FOR UPDATE in your application. Update cursors
use promotable locks; that is, the database server places
an update lock (meaning other users can still view the row) when the
application fetches the row, but the lock is changed to an exclusive
lock when the application updates the row using an update cursor and
UPDATE...WHERE CURRENT OF.
The advantage of using an update cursor is that you can view the
row with the confidence that other users cannot change it or view
it with an update cursor while you are viewing it and before you update
it.
Tip: In an ANSI-compliant database, update cursors
are unnecessary because any select cursor behaves the same as an update
cursor.
The pseudocode in the following figure shows when the database
server places and releases locks with a cursor. Figure 1: Locks Placed for Update
