1 / 93

Object Oriented Database Management

Object Oriented Database Management. Outline. Motivation Embedding SQL in host language Object Data Model Persistent Programming Languages Object Query Language Object-orientation in SQL. Motivation of ODBMSs. Application data structures.

tao
Download Presentation

Object Oriented Database Management

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Object Oriented Database Management

  2. Outline • Motivation • Embedding SQL in host language • Object Data Model • Persistent Programming Languages • Object Query Language • Object-orientation in SQL

  3. Motivation of ODBMSs Application data structures • Complex objects in emerging DBMS applications cannot be effectively represented as records in relational model. • Representing information in RDBMSs requires complex and inefficient conversion into and from the relational model to the application programming language • ODBMSs provide a direct representation of objects to DBMSs overcoming the impedance mismatch problem Copy and translation Transparent ODBMS data transfer Relational representation RDBMS

  4. Embedded SQL • Access to database from a general purpose programming language required since: • Not all queries can be expressed in SQL --e.g., recursive queries cannot be written in SQL. • Non declarative actions -- e.g., printing reports cannot be done from SQL. • General purpose language in which SQL is embedded called host language. • SQL structures permitted in host language called embedded SQL. C compiler SQL library calls + C SQL+ C pre- compiler .o file loader SQL library object code Embedded SQL Compilation

  5. Embedded SQL • SQL commands embedded in the host programming language • Data exchanged between host language and DBMS using cursors • SQL query passed from host language to DBMS which computes the answer set • A cursor can be viewed as a pointer into the answer set • DBMS returns the cursor to the programming language • Programming language can use the cursor to get a record at a time access to materialized answer.

  6. Example of Embedded SQL :dname = “toy”; raise = 0.1; EXEC SQL SELECT dnum into :dnum FROM Department WHERE dname= :dname; EXEC SQL DECLARE Emp CURSORFOR SELECT * FROM Employee WHERE dno = :dnum FOR UPDATE EXEC SQL OPEN Emp; EXEC SQL FETCH Emp INTO :E.ssn, :E.dno, :E.name, :E.sal; while (SQLCODE == 0) { EXEC SQL UPDATE WHERE CURRENT OF CURSOR SET sal = sal * (1 + ::raise); EXEC SQL FETCH Emp INTO :E.ssn, :E.dno, :E.name, :E.sal; } EXEC SQL CLOSE CURSOR Emp /* SQL embedded in C to read the list of employees who work for the toy department and give them a 10 percent raise */

  7. Object Oriented Database Management • Object Oriented databases have evolved along two different paths: • Persistent Object Oriented Programming Languages: (pure ODBMSs) • Start with an OO language (e.g., C++, Java, SMALLTALK) which has a rich type system • Add persistence to the objects in programming language where persistent objects stored in databases • Object Relational Database Management Systems (SQL3 Systems) • Extend relational DBMSs with the rich type system and user-defined functions. • Provide a convenient path for users of relational DBMSs to migrate to OO technology • All major vendors (e.g., Informix, Oracle) will/are supporting features of SQL3.

  8. Object Database Management Group (ODMG) • Special interest group to develop standards that allow ODBMS customers to write portable applications • Standards include: • Object Model • Object Specification Languages • Object Definition Language (ODL) for schema definition • Object Interchange Format (OIF) to exchange objects between databases • Object Query Language • declarative language to query and update database objects • Language Bindings (C++, Java, Smalltalk) • Object manipulation language • Mechanisms to invoke OQL from language • Procedures for operation on databases and transactions

  9. Object Model • Object: • observable entity in the world being modeled • similar to concept to entity in the E/R model • An object consists of: • attributes: properties built in from primitive types • relationships: properties whose type is a reference to some other object or a collection of references • methods: functions that may be applied to the object.

  10. Class • Similar objects with the same set of properties and describing similar real-world concepts are collected into a class. • Class definition: interface Employee { attribute string name; attribute integer salary; attribute date date-of-birth; attribute integer empid; relationship Projects works-for inverse Projects::team; age-type age(); } Interface Projects{ attribute string name; attribute integer projid; relationship Employee team inverse Emplolyee works-for; int number-of-employees(); }

  11. Class Extents • For each ODL class, an extent may be declared. • Extent is the current set of objects belonging to the class. • Similar notion to the relation in the relational model. • Queries in OQL refer to the extent of a class and not the class directly. interface Employee (extent Emp-set) { attribute string name; attribute integer salary; attribute date date-of-birth; attribute integer empid; relationship Projects works-for inverse Projects::team; age-type age(); }

  12. Subclasses and Inheritance • A class can be declared to be a subclass of another class. • Subclasses inherit all the properties • attributes • relationships • methods from the superclass. Interface Married-Employee: Employees { string spouse-name; } • Substitutability: any method of superclass can be invoked over objects of any subclass (code reuse)

  13. Class Hierarchy person student employee undergrad student assistant grad staff faculty RA TA

  14. Multiple Inheritance • A class may have more than one superclass. • A class inherits properties fromeach of its superclasses. • There is a potential of ambiguity -- variable with same name inherited from two superclasses: • flag and error • rename variable • choose one

  15. Object Identity • Each object has an identity which it maintains even if some or all of its attributes change. • Object identity is a stronger notion of identity than in relational DBMSs. • Identity in relational DBMSs is value based (primary key). • Identity in ODBMSs built into data model • no user specified identifier is required • OID is a similar notion as pointer in programming language • Object identifier (OID) can be stored as attribute in object to refer to another object. • References to other objects via their OIDs can result in a containment hierarchy • Note: containment hierarchy different from class hierarchy

  16. Containment Hierarchy bicycle wheel brake gear frame tire rim spoke lever pad Links in containment hierarchy should be read as is-part-of instead of is-a

  17. Persistence • Objects created may have different lifetimes: • transient: allocated memory managed by the programming language run-time system. • E.g., local variables in procedures have a lifetime of a procedure execution • global variables have a lifetime of a program execution • persistent: allocated memory and stored managed by ODBMS runtime system. • Classes are declared to be persistence-capable or transient. • Different languages have different mechanisms to make objects persistent: • creation time: Object declared persistent at creation time (e.g., in C++ binding) (class must be persistent-capable) • persistence by reachability: object is persistent if it can be reached from a persistent object (e.g., in Java binding) (class must be persistent-capable).

  18. Persistent Object-Oriented Programming Languages • Persistent objects are stored in the database and accessed from the programming language. • Classes declared in ODL mapped to the programming language type system (ODL binding). • Single programming language for applications as well as data management. • Avoid having to translate data to and from application programming language and DBMS • efficient implementation • less code • Programmer does not need to write explicit code to fetch data to and from database • persistent objects to programmer looks exactly the same as transient objects. • System automatically brings the objects to and from memory to storage device. (pointer swizzling).

  19. Disadvantages of ODBMS Approach • Low protection • since persistent objects manipulated from applications directly, more changes that errors in applications can violate data integrity. • Non-declarative interface: • difficult to optimize queries • difficult to express queries • But ….. • Most ODBMSs offer a declarative query language OQL to overcome the problem. • OQL is very similar to SQL and can be optimized effectively. • OQL can be invoked from inside ODBMS programming language. • Objects can be manipulated both within OQL and programming language without explicitly transferring values between the two languages. • OQL embedding maintains simplicity of ODBMS programming language interface and yet provides declarative access.

  20. OQL Example interface Employee { attribute string name; relationship setof(Projects) works-for inverse Projects::team; } Interface Projects{ attribute string name; relationship setof(Employee) team inverse Emplolyee works-for; int number-of-employees(); } Select number-of-employees() From Employee e, e.works-for where name = “sharad” Find number of employees working on each project “sharad” works on

  21. Migration of RDBMSs towards OO Technologies • SQL3 standard incorporates OO concepts in the relational model. • A row in a table considered as an object • SQL3 allows a type to be declared for tuples (similar to class in ODBMSs) • Relations are collection of tuples of a row type (similar to extent in ODBMSs) • Rows in a relation can refer to each other using a reference type (similar to object identity in ODBMSs) • A reference can be dereferenced to navigate among tables • Attributes in a relation can belong to abstract data types • Methods and functions (expressed in SQL as well as host programming language) can be associated with abstract data types

  22. SQL-3 Example CREATE ROW TYPE Employee-type { name CHAR(30) works-for REF(Projects-type) } CREATE ROW TYPE Projects-type { name CHAR(30) team setof(REF(Employee-type)) } CREATE TABLE Emp OF TYPE Employee-type CREATE TABLE Project of TYPE Project-type Select works-for --> name From Emp Where name = ‘sharad’ Return name of the project sharad works for

  23. OQL CMSC-461 Database Management Systems

  24. OQL -- Motivation • Relational languages suffer from impedance mismatch when we try to connect them to conventional languages like C or C++. • The data models of C and SQL are radically different, e.g. C does not have relations, sets, or bags as primitive types; C is tuple-at-a-time, SQL is relation-at-a-time.

  25. OQL -- Motivation (II) • OQL is an attempt by the OO community to extend languages like C++ with SQL-like, relation-at-a-time dictions. • OQL is query language paired with schema-definition language ODL.

  26. OQL Types • Basic types: strings, ints, reals, etc., plus class names. • Type constructors: • Struct for structures. • Collection types: set, bag, list, array. • Like ODL, but no limit on the number of times we can apply a type constructor. • Set(Struct()) and Bag(Struct()) play special roles akin to relations.

  27. OQL Uses ODL as its Schema-Definition Portion • For every class we can declare an extent = name for the current set of objects of the class. • Remember to refer to the extent, not the class name, in queries.

  28. Example • interface Bar (extent Bars){ attribute string name; attribute string addr; relationship Set<Sell> beersSold inverse Sell::bar;}

  29. Example (II) • interface Beer (extent Beers){ attribute string name; attribute string manf; relationship Set<Sell> soldBy inverse Sell::beer;}

  30. Example (III) • interface Sell (extent Sells){ attribute float price; relationship Bar bar inverse Bar::beersSold; relationship Beer beer inverse Beer::soldBy;}

  31. Path Expressions • Let x be an object of class C. • If a is an attribute of C, then x.a = the value of a in the x object. • If r is a relationship of C, then x.r = the value to which x is connected by r. • Could be an object or a collection of objects, depending on the type of r. • If m is a method of C , then x.m (...) is the result of applying m to x.

  32. Examples • Let s be a variable whose type is Sell. • s.price = the price in the object s. • s.bar.addr = the address of the bar mentioned in s . • Note: cascade of dots OK because s.bar is an object, not a collection.

  33. Example of Illegal Use of Dot • b.beersSold.price, where b is a Bar object. • Why illegal? Because b.beersSold is a set of objects, not a single object.

  34. OQL Select-From-Where • SELECT < list of values >FROM < list of collections and typical members >WHERE < condition >

  35. OQL Select-From-Where (II) • Collections in FROM can be:1. Extents.2. Expressions that evaluate to a collection. • Following a collection is a name for a typical member, optionally preceded by AS.

  36. Example • Get the menu at Joe's. SELECT s.beer.name, s.price FROM Sells s WHERE s.bar.name = "Joe's Bar" • Notice double-quoted strings in OQL. • Result is of type Bag(Struct(name: string, price: float))

  37. Example • Another way to get Joe's menu, this time focusing on the Bar objects. SELECT s.beer.name, s.price FROM Bars b, b.beersSold s WHERE b.name = "Joe's Bar" • Notice that the typical object b in the first collection of FROM is used to help define the second collection. • Typical usage: if x.a is an object, you can extend the path expression; if x.a is a collection, you use it in the FROM list.

  38. Tailoring the Type of the Result • Default: bag of structs, field names taken from the ends of path names in SELECT clause. • Example SELECT s.beer.name, s.price FROM Bars b, b.beersSold s WHERE b.name = "Joe's Bar"has result type: Bag(Struct( name: string, price: real))

  39. Rename Fields • Prefix the path with the desired name and a colon. • Example SELECT beer: s.beer.name, s.price FROM Bars b, b.beersSold s WHERE b.name = "Joe's Bar"

  40. Change the Collection Type • Use SELECT DISTINCT to get a set of structs.

  41. Example • SELECT DISTINCT s.beer.name, s.priceFROM Bars b, b.beersSold sWHERE b.name = "Joe's Bar" • Use ORDER BY clause to get a list of structs.

  42. Example • joeMenu =SELECT s.beer.name, s.priceFROM Bars b, b.beersSold sWHERE b.name = "Joe's Bar"ORDER BY s.price ASC • ASC = ascending (default); DESC = descending. • We can extract from a list as if it were an array, e.g. cheapest = joeMenu[1].name;

  43. Subqueries • Used mainly in FROM clauses and with quantifiers EXISTS and FORALL.

  44. Example: Subquery in FROM • Find the manufacturers of the beers served at Joe's.SELECT b.manfFROM (SELECT s.beerFROM Sells sWHERE s.bar.name = "Joe's Bar") b

  45. Quantifiers • Boolean-valued expressions for use in WHERE-clauses.FOR ALL x IN < collection > : < condition >EXISTS x IN < collection > : < condition > • The expression has value TRUE if the condition is true for all (resp. at least one) elements of the collection.

  46. Example • Find all bars that sell some beer for more than $5. SELECT b.name FROM Bars b WHERE EXISTS s IN b.beersSold : s.price > 5.00 • ProblemHow would you find the bars that only sold beers for more than $5?

  47. Example • Find the bars such that the only beers they sell for more than $5 are manufactured by Pete's. SELECT b.name FROM Bars b WHERE FOR ALL be IN ( SELECT s.beer FROM b.beersSold s WHERE s.price > 5.00 ) : be.manf = "Pete's"

  48. Extraction of Collection Elements • a) A collection with a single member: Extractthe member with ELEMENT.

  49. Example • Find the price Joe charges for Bud and put the result in a variable p. • p = ELEMENT( SELECT s.price FROM Sells s WHERE s.bar.name = "Joe's Bar" AND s.beer.name = "Bud" )

  50. Extraction of Collection Elements (II) • b) Extracting all elements of a collection, one at a time: • 1. Turn the collection into a list. • 2. Extract elements of a list with <list name>[i].

More Related