Application A wants to read a subset of rows from table TAB1 multiple times. Which of the
following isolation levels should Application A use to prevent other users from making
modifications and additions to table TAB1 that will affect the subset of rows read?
A.
Repeatable Read
B.
Read Stability
C.
Cursor Stability
D.
Uncommitted Read
The read stability isolation level locks only those rows that an application retrieves during a unit of work. RS ensures that any qualifying row read during a UOW cannot be changed by other application processes until the UOW completes, and that any row changed by another application process cannot be read until the change is committed by that process. Under RS, access to uncommitted data and non-repeatable reads are not possible. However, phantom reads are possible.
This isolation level ensures that all returned data remains unchanged until the time the application sees the data, even when temporary tables or row blocking is used.
The RS isolation level provides both a high degree of concurrency and a stable view of the data. To that end, the optimizer ensures that table-level locks are not obtained until lock escalation occurs.
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Repeatable read
The Repeatable Read (RR) isolation level ensures:
Any row read during a unit of work is not changed by other activation groups that use different commitment definitions until the unit of work is complete.
Any row changed (or a row that is currently locked with an UPDATE row lock) by another activation group using a different commitment definition cannot be read until it is committed.
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