ASA SQL User's Guide
Monitoring and Improving Performance
Top performance tips
Specifying the correct cursor type can improve performance. For example, if a cursor is read-only, then declaring it as read-only allows for faster optimization and execution, since there is less material to build (no check constraints, and so on). If the cursor is updateable, some rewrites can be skipped. Also, if a query is updateable, then depending on the execution plan chosen by the optimizer, the execution engine must use a keyset driven approach. Keep in mind that key-set cursors are more expensive.