Optimistic Concurrency Control (cont)
OCC prevents:
T reading dirty data, T overwriting Ti's changes
Problems with OCC:
- increased roll backs**
- tendency to roll back "complete" tx's
- cost to maintain S,V,F sets
** "Roll back" is relatively cheap
- changes to data are purely local before Writing phase
- no requirement for logging info or undo/redo (see later)
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