Object-Based Databases

Object-Based Databases

1. Overview

   • The first obstacle faced by programmers using the relational data model was the limited type system supported by the relational model.
   • domains require correspondingly complex data types, such as nested record structures, multivalued attributes, and inheritance, which are supported by traditional programming languages.
   • query languages, in particular SQL, need to be correspondingly extended to deal with the richer type system.

2. Complex Data Types

   • The basic data items are records that are fairly small and whose fields are atomic — that is, they are not further structured, and first normal form holds
   • The alternative of normalization by creating a new relation is expensive

3. Structured Types and Inheritance in SQL

   • Structured Types
     • Structured types allow composite attributes of E-R designs to be represented directly.
     • The components of a composite attribute can be accessed using a “dot” notation
     • An alternative way of defining composite attributes in SQL is to use unnamed row types.
     • The variable self refers to the Person instance on which the method is invoked.
   • Type Inheritance
     • We can do this if the type system supports multiple inheritance,wherea type is declared as a subtype of multiple types.
     • Note that the SQL standard does not support multiple inheritance, ​although future versions of the SQL standard

4. Table Inheritance

   • Subtables in SQL correspond to the E-R notion of specialization/generalization.
   • The only keyword can also be used in delete and update statements. Without the only keyword, a delete statement on a supertable, such as people, also deletes tuples that were originally inserted in subtables
   • tuples that were inserted in subtables are not affected, even if they satisfy the where clause conditions.
   • Each tuple of the supertable can correspond to at most one tuple in each of its immediate subtables.
   • SQL has an additional constraint that all the tuples corresponding to each other must be derived from one tuple (inserted into one table).

5. Array and Multiset Types in SQL

   • SQL supports two collection types: arrays and multisets; array types were added
   • evaluating to a collection can appear anywhere that a relation name may appear, such as in a from clause, as the following paragraphs illustrate.
     • The transformation of a nested relation into a form with fewer (or no) relationvalued attributes is called unnesting.

6. Implementing O-R Features

   • Object-relational database systems are basically extensions of existing relational database systems.
   • Each table stores the primary key (which may be inherited from a parent table) and the attributes that are defined locally.
   • Each table stores all inherited and locally defined attributes. When a tuple is inserted, it is stored only in the table in which it is inserted, and its presence is inferred in each of the supertables.

7. Persistent Programming Languages

   • Database languages differ from traditional programming languages in that they directly manipulate data that are persistent— that is, data that continue to exist even after the program that created it has terminated.
     • The code to convert between objects and tuples operates outside the object-oriented type system, and hence has a higher chance of having undetected errors.
     • Conversion between the object-oriented format and the relational format of tuples in the database takes a substantial amount of code.

8. Object-Relational Mapping

   • systems provide a third approach to integration of object-oriented programming languages and databases.
   • Object-relational mapping systems also provide query languages that allow programmers to write queries directly on the object model

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