Getting Started with DB2 Express-C

220 Getting Started with DB2 Express-C
Figure 13.7 – Phantom read
Figure 13.7 shows the following sequence of events:
1. App1 opens a cursor
2. App2 adds a row to the database that would qualify for the cursor
3. App2 commits changes
4. App1 closes and reopens cursor
In this case, App1 would not get the same data on a repeated read, it would get more rows,
that’s why this problem is called “phantom read”.
13.4 Isolation Levels
You can think of isolation levels as locking policies where, depending on the isolation level
chosen, you may get different behaviors for database locking with an application.
DB2 provides different levels of protection to isolate data:
• Uncommitted Read (UR)
• Cursor Stability (CS)
• Read Stability (RS)
• Repeatable Read (RR)
13.4.1 Uncommitted read
Uncommitted read is also known as dirty read. It is the lowest level of isolation, and
provides the highest degree of concurrency. No row locks are obtained on read operations,
unless another application attempts to drop or alter a table; and update operations act as if
using the cursor stability isolation level.