Using Cursors

Cursors are very useful for handling large amounts of data but there are a number of issues which you should bear in mind when using cursors, namely: data concurrency, integrity and consistency.

To ensure the integrity of your data, a database server can implement different locking methods. Some types of data access do not acquire any locks, some acquire a shared lock and some an exclusive lock. A shared lock allows other processes to access the data but not update it. An exclusive lock does not allow any other process to access the data.

When using cursors there are three levels of isolation and these control the data that a cursor can read and lock:

It is worth pointing out that there can be problems with deadlocks or "deadly embraces" where two processes are competing for the same data. The classic example is where one process locks data A and then requests a lock on data B while a second process locks data B and then requests a lock on data A. Both processes have data that the other process requires. The database server should spot this case and send errors to one, or both, processes.


Oracle, Sybase and Informix allow an application to set the isolation level of the cursor and their documentation discusses the types of locks that are applied and how they work. Their documentation also discusses the physical level that the data is locked at. This can be a single row, a set of rows (that is, the page level), or the whole table. Care should be taken when using cursors that scan multiple tables, or tables that are used by most processes, as this will reduce the accessibility of the locked data.



Oracle, Sybase and Informix allow cursors to be defined with a number of different clauses, for example FOR READ ONLY, FOR UPDATE etc. These clauses effect the isolation level of the cursor and how it will act when involved in transaction processing. For more information on the effect of these difference clauses, refer to the SQL reference book supplied with your database.