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Extra info for ACTUALTESTS Oracle 1Z0-032 Exam Q and A 05 30 05
2-2 by a where histories H, - H,, belong. 7). if neither is contained in the other. 36 CHAPTER 2 I SERIALIZABILITY THEORY ~- - All histories FIGURE 2-2 Relationships between Histories that are SR, IX, ACA, and ST Figure 2-2 illustrates that there exist histories that are SR but not RC. In a DBS that must correctly handle transaction and system failures (as most must do), the scheduler must enforce recoverabihty (or the even stronger properties of cascadeIessness or strictness) in addition to serializability.
2 SERIALIZABLE w,[xl - H, = ~*bl + 7,[Yl \ / w,[xl - Cl r2[z3- Ll\ w&l - w[xl , rIbI, H, = / 31 ’ c2 / 7,[xl HISTORIES 72[zl - AYI ’ T WL[YIy w2bl c’ ’ c2 FlGURE 2-l Example Histories H2 and H3 are equivalent, but H4 is not equivalent to either. The idea underlying this definition is that the outcome of a concurrent execution of transactions depends only on the relative ordering of conflicting operations. To see this observe that executing two nonconflicting operations in either order has the same computational effect.
To see the relationship between the two notations, consider the following transaction. ~Note that we generally do not draw arcs implied by transitivity. For example, the arc TJX] --t c, is implied by TJX] + w,[z] and w,[z] + cZ. Our formal definition of a transaction does not capture every observable aspect of the transaction execution it models. For example, it does not describe the initial values of data items or the values written by Writes. Moreover, it only describes the database operations, and not, for example, assignment or conditional statements.