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CHAPTER 12 DATA REDUNDANCY AND DATABASE DESIGN
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Managing Redundancy
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The fact that the design of Example 12 from the previous section is redundant is clearly shown by the fact that it satisfies the following constraint (C12): CONSTRAINT C12 TOTALS = SUMMARIZE PAYMENTS BY { CUST# } ADD ( SUM ( AMT ) AS TOTAL ) ; There are four basic approaches to dealing with the kind of redundancy illustrated by this example: 1. Raw design only 2. Declare the constraint 3. Use a view 4. Use a snapshot Let s take a closer look.
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1. Raw Design Only
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This is the approach most likely to be encountered in practice, given the limited functionality provided by most of today s DBMS implementations. The idea is simply that: a. Relvars PAYMENTS and TOTALS are defined exactly as shown in the previous section. b. Constraint C12 is not declared to the DBMS. c. Maintaining the derived data is the user s responsibility one hundred percent. (Or some user s responsibility, at any rate; the maintenance might be done by means of a triggered procedure, but some user still has to write that procedure.) In effect, this approach trades off (a) the extra work involved on the part of the user in executing certain updates (as well as the associated performance hit) against (b) the improved performance obtained when executing certain queries. But there are no guarantees if the user makes a mistake during some update that (in effect) causes Constraint C12 to be violated, well, tough.
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2. Declare the Constraint
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In this approach Constraint C12 is explicitly declared to the DBMS and the DBMS takes the responsibility for enforcing it. Maintaining the derived data is still the user s responsibility, though. What s more, if the user carries out this task reliably and correctly, the constraint checking will never fail, and it will thus, in effect, constitute pure overhead on the user s updates. But we can t dispense with the constraint, of course, precisely because we need the system to check that the user is carrying out the maintenance task reliably and correctly. By the way, proper support for this approach will require the system to support, and the user to use, multiple assignment in the maintenance task (see 11).
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CHAPTER 12 DATA REDUNDANCY AND DATABASE DESIGN
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3. Use a View
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Clearly it would be better if, instead of simply declaring the constraint, we could actually inform the system of the rule by which the derived data is defined and have the system perform the derivation process automatically. And we can; that s exactly what the view mechanism does. To be specific, we can replace the base relvar TOTALS by a view of the same name, thus: VAR TOTALS VIRTUAL ( SUMMARIZE PAYMENTS BY { CUST# } ADD ( SUM ( AMT ) AS TOTAL ) ) ; Now the user no longer has to worry about maintaining the derived data; moreover, there s now no way that Constraint C12 can possibly be violated, and there s no need even to state it any more, except perhaps informally (as a means of telling the user the semantics of the view, perhaps). Note, however, that the user does have to be explicitly told not to try to maintain the totals! This fact doesn t mean the user has to be told that relvar TOTALS is a view, though; it just means the user has to be told that the maintenance task will effectively be performed by the system.
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4. Use a Snapshot
The drawback to the view solution, however, is that the derivation process is performed every time the view is referenced. Thus, if the object of the exercise is (in effect) to do that work at update time in order to improve subsequent query performance, the view solution is clearly inadequate. In that case, we should use a snapshot instead of a view: VAR TOTALS SNAPSHOT ( SUMMARIZE PAYMENTS BY { CUST# } ADD ( SUM ( AMT ) AS TOTAL ) ) REFRESH ON EVERY UPDATE ; The snapshot concept has its origins in a paper by Michel Adiba ( Derived Relations: A Unified Mechanism for Views, Snapshots, and Distributed Data, Proc. 1981 Int. Conf. on Very Large Data Bases, Cannes, France, September 1981; see also the earlier version Database Snapshots, by Michel E. Adiba and Bruce G. Lindsay, IBM Research Report RJ2772, March 7th, 1980). Basically, snapshots, like views, are derived relvars; unlike views, however, they re real, not virtual that is, they re represented not just by their definition in terms of other relvars, but also (at least conceptually) by their own separately materialized copy of the data.24 In other words, defining a snapshot is much like executing a query, except that: a. The result of the query is kept in the database under the specified name (TOTALS in the example) as a read-only relvar (read-only, that is, apart from the periodic refresh see point b. immediately following). b. Periodically (ON EVERY UPDATE in the example) the snapshot is refreshed that is, its current value is discarded, the query is executed again, and the result of that new execution becomes the new snapshot value.
24. As I mentioned earlier in this chapter, snapshots are also referred to (at least in some circles) as materialized views. In my opinion, this terminology should be firmly resisted. A large part of the point about views, at least as far as the relational model is concerned, is precisely that they aren t materialized in other words, they re virtual and materialized view is simply a contradiction in terms.
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