tailieunhanh - An Introduction to Database Systems 8Ed - C J Date - Solutions Manual Episode 2 Part 8

Tham khảo tài liệu 'an introduction to database systems 8ed - c j date - solutions manual episode 2 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Is an object DBMS really a DBMS Self-explanatory. But the point perhaps is this Object DBMSs do surely have a role to play there are surely problems out there for which an object DBMS is the right solution. No argument here. No The argument rather is simply that those DBMSs are not for all kinds of reasons DBMSs in the sense in which the database community understands and uses that term. It might have been better not to call them DBMSs. Reject the jingle persistence orthogonal to type Summary For this chapter alone out of the whole book it seems worth including most of the summary section in these notes because it really serves not just as a summary per se but also as a critical analysis of the material discussed and as a lead-in to what might constitute a good object model. So here goes the following is reworded just a little from the original Begin quote Object classes . types Obviously essential indeed they re the most fundamental construct of all . Objects Objects themselves both mutable and immutable are clearly essential though I d prefer to call them simply variables and values respectively. Actually it might be argued that mutable objects aren t quite the same thing as variables in the classical sense. The one operator that must be available for a variable V is assignment to V it s precisely the availability of that operator that makes V variable But objects aren t required to have an associated assignment method and indeed they typically don t instead such a method exists only if the class definer defines it. Object IDs Unnecessary and in fact undesirable at the model level that is because they re basically just pointers. Note too the argument elaborated in the next chapter that OIDs are fundamentally incompatible with a good model of inheritance. One problem not the only one is that Copyright c 2003 C. J. Date page OIDs lead to the possibility of shared variables a possibility that doesn t exist nor do we want it to in the relational world.