Compiling State Constraints
نویسندگان
چکیده
The evaluation of constraints needs in many cases that a large portion of the database to be accessed. This makes the process of integrity checking very di cult to implement e ciently. Thus nding methods to improve the evaluation of integrity constraints is an important area of research in the eld of database integrity. Most of these methods are based on simpli cation principles. One of these methods is presented by Nicolas in [Nic82]. In this method the simpli ed form of a constraint depends on updating operations performed on database states. For that reason, the simpli ed form is obtained at update time. In this report we show that, for a given constraintW and an update that is to be performed to a relation R, it is not necessary to do all the steps of the method at run time, but we can do most of these steps at compile time. We do that by developing a representation that stores simpli ed instances of W together with other information about occurrences of R in W into meta relations. The simpli ed instances stored in the meta relations are obtained form W by applying the same simpli cation steps of the method, but here we use generic constants instead of speci c update values. When an update is performed to the relation R, the generic constants in the meta relations are replaced with the update values and a relational algebra expression is performed on the obtained relation, resulting in a set of formulas. We will show that by only applying the third step of the method to the conjunctions of these formulas we can get the simpli ed form obatined by the simpli cation method at run time.
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