Chapter 10 - 1 Nested Schemes Flat schemes often have replicated data values. Nested schemes allow us to collapse some of these replicated data values.

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Presentation transcript:

Chapter Nested Schemes Flat schemes often have replicated data values. Nested schemes allow us to collapse some of these replicated data values. NrBeds RoomNr NrBeds ( RoomNr )*

Chapter Redundancy in Nested Schemes The redundancy definition is the same as for flat relations. If a value change causes a constraint violation, the value is redundant. NrBeds (RoomNr (View)* )* 2 1 Sea Forest City 2 Sea Forest 3 City View (RoomNr NrBeds)* Sea Forest City

Chapter Algorithm 10.3 Input: a canonical, acyclic, binary ORM hypergraph. Output: a set of nested schemes with no potential redundancy. Repeat Mark an unmarked node in as the first attribute in a new nested scheme. While an unmarked edge is incident on a marked node A: Mark the edge. If A  B: Add B with A; Mark B. If A  B: Add B with A; Mark B if all B’s incident edges are marked. If A  B: Nest B under A; Mark B. Else (A – B): Nest B under A; Mark B if all B’s incident edges are marked. Until all nodes have been marked

Chapter Nested Scheme Generation Example 1. NrBeds (RoomNr, RoomName, Cost (View)* (GuestNr, GuestName)* )* 2. RoomNr, RoomName, Cost, NrBeds (View)* (GuestNr, GuestName)* 3. GuestNr GuestName RoomNr RoomNr, RoomName, Cost, NrBeds (View)*

Chapter Redundancy Prevention xa1yb2zxa1yb2z A ( BC )* ax1 y1 by1 z2 This replication... … causes this redundancy.

Chapter Generalization of Algorithm 10.3 for N-ary Relationship Sets “Composite nodes” can be treated as a node (in Algorithm 10.3). –B C (A)* (D)* –D (B C)*; A B C NNF (see Exercise 10.35), basically: –Schemes should be constructed along hypergraph paths. –Schemes should not violate the natural 1-many hierarchical structure.

Chapter Guidelines for Selecting Nested Schemes Select “important nodes” as the initial nodes for nested-scheme generation – e.g., Scheme 3 or 2 in earlier Bed-&-Breakfast example. Maximize the size of schemes. –Select nodes included in the largest number of FD closures (i.e., when Algorithm 10.3 requires a new node to be arbitrarily selected, compute the set of unmarked nodes in the FD closure of every unmarked node and choose a node included in at least as many sets as any other node) – e.g., Scheme 1 in earlier example. –When possible, adjust these generated maximal schemes by placing the most important node first – e.g., Scheme 2 in earlier example.

Chapter Cost Analysis for Nested Schemes Nested schemes impose variable-length records. Recall variable-length record implementation strategies: –Reserve enough space for maximum. –Chain each nested record. –Reserve space for the expected number and chain the rest. Insertion, deletion, modification, retrieval tradeoffs.