Block cursors and lookahead cursors are both associated with NO-LOCK queries. These are queries where the lock status has been explicitly set to NO-LOCK or where the lock status has been explicitly set to SHARE-LOCK and the transaction isolation level is set to read-uncommitted. Block cursors and lookahead cursors are mutually exclusive.

Block cursors are more efficient than lookahead cursors for the following reasons:

• The results of a query are bound directly to the area from which they are copied into the record buffer on the client. Lookahead cursors copy out of a common bind area into the lookahead cache and then are copied into client record buffer resulting in multiple copies of the data.
• The fetch process used by block cursors fetches multiple rows at a time, reducing the number of driver calls and potentially reducing the total number of network round trips to the server.
• Block cursor memory is preallocated prior to fetching a result set, whereas lookahead cursors post allocate memory as records are fetched from the result set.
• Block cursors are directly linked to the result set binding feature whereas lookahead cursors have no particular dependency on whether binding or late-binding is active.