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, such as check constraints, and so on. If the cursor is updatable, some query rewrites can be skipped. Also, if a query is updatable, then depending on the execution plan chosen by the optimizer, the database server must use a keyset driven approach. Keep in mind that keyset cursors are more expensive. See Choosing cursor types.
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