read-only-clause
The READ ONLY clause indicates that the result table is read-only; therefore, the cursor cannot be used for positioned UPDATE and DELETE statements.
Some result tables are read-only by nature. (For example, a table based on a read-only view). FOR READ ONLY can still be specified for such tables, but the specification has no effect.
For result tables in which updates and deletes are allowed, specifying FOR READ ONLY can possibly improve the performance of FETCH operations by allowing the database manager to do blocking and avoid exclusive locks. For example, in programs that contain dynamic SQL statements without the FOR READ ONLY or ORDER BY clause, the database manager might open cursors as if the UPDATE clause was specified.
A read-only result table must not be referred to in an UPDATE or DELETE statement, whether it is read-only by nature or specified as FOR READ ONLY.
To guarantee that selected data is not locked by any other job, you can specify the optional syntax of USE AND KEEP EXCLUSIVE LOCKS on the isolation-clause. This guarantees that the selected data can later be updated or deleted without incurring a row lock conflict.
Syntax Alternatives: FOR FETCH ONLY can be specified in place of FOR READ ONLY.