Cursors cannot use the dynamic index of row IDs generated by the OR strategy. Queries that use the OR strategy in standalone select statements usually perform table scans using read-only cursors. Updatable cursors may need to use a unique index and still require access to each data row, in sequence, in order to evaluate the query clauses.
See “Access Methods and Costing for or and in Clauses” for more information.
A read-only cursor using union creates a worktable when the cursor is declared, and sorts it to remove duplicates. Fetches are performed on the worktable. A cursor using union all can return duplicates and does not require a worktable.