Which of the following is TRUE for the DB2 isolation level Cursor Stability (CS)?
A.
An application process acquires at least a share lock on the current row of every cursor.
B.
Any row that is read during a unit of work cannot be changed by other application processes
until the unit of work is complete.
C.
Any row changed by another application process can be read, even if the change has not been
committed by that application process.
D.
An application process that issues the same query more than once in a unit of work will not see
additional rows caused by other application processes appending new information to the
database.
Under CS, access to the uncommitted data of other applications is not possible. However, non-repeatable reads and phantom reads are possible.
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Cursor stability (CS)
The cursor stability isolation level locks any row being accessed during a transaction while the cursor is positioned on that row. This lock remains in effect until the next row is fetched or the transaction terminates. However, if any data in the row was changed, the lock is held until the change is committed.
Under this isolation level, no other application can update or delete a row while an updatable cursor is positioned on that row. Under CS, access to the uncommitted data of other applications is not possible. However, non-repeatable reads and phantom reads are possible.
CS is the default isolation level. It is suitable when you want maximum concurrency and need to see only committed data.
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When a transaction using the cursor stability isolation level retrieves a row from a table via a cursor, no other transaction can update or delete that row while the cursor is positioned on it. However, other transactions can add new rows to the table as well as perform update or delete operations on rows positioned on either side of the locked row – provided that the locked row
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DB2 9 Fundamentals exam 730 prep, Part 6: Data concurrency Page 10 of 23
itself wasn’t accessed using an index. Once acquired, the lock remains in effect until the cursor is repositioned or until the owning transaction is terminated. (If the cursor is repositioned, the lock being held on the previous row read is released and a new lock is acquired for the row the cursor is now positioned on.) Furthermore, if the owning transaction modifies any row it retrieves, no other transaction is allowed to update or delete that row until the owning transaction is terminated, even though the cursor may no longer be positioned on the modified row. As with the repeatable read and read stability isolation levels, transactions using the cursor stability isolation level (which is the default isolation level used) won’t see changes made to other rows by other transactions until those changes have been committed.
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