Topic 3 Object Relational Database

Transcription

1 Topic 3 Object Relational Database

2 Limitation of Relational Data Model Uniformity Large number of similarly structure data Record orientation Basic data consists of fixed length records Small data items Each record is short Atomic fields No structure with a field. First normal form holds

3 Need for Complex Data Types Complex data types have grown more important in recent years E.g. Addresses can be viewed as a Single string, or Separate attributes for each part, or Composite attributes (which are not in first normal form) E.g. it is often convenient to store multivalued attributes as-is, without creating a separate relation to store the values in first normal form

4 Requirements of New application CAD, CASE,, Multimedia databases Office information systems Hypertext databases

5 OO Data Model Review Object, complex object Set of variables, messages, methods Object Identity, Value, Name, Built-in as OID Object Class,. relationship, complex attributes,. Inheritance, Class hierarchy Multiple inheritance Encapsulation,

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8 Object-Oriented Languages Object-oriented concepts can be used in different ways Object-orientation can be used as a design tool, and be encoded into, for example, a relational database analogous to modeling data with E-R diagram and then converting to a set of relations), like UML The concepts of object orientation can be incorporated into a programming language that is used to manipulate the database. Object-relational systems add complex types and object-orientation to relational language. Persistent programming languages extend object-oriented programming language to deal with databases by adding concepts such as persistence and collections.

9 Persistent Programming Languages Persistent Programming languages allow objects to be created and stored in a database, and used directly from a programming language allow data to be manipulated directly from the programming language No need to go through SQL. No need for explicit format (type) changes format changes are carried out transparently by system Without a persistent programming language, format changes becomes a burden on the programmer More code to be written More chance of bugs allow objects to be manipulated in-memory no need to explicitly load from or store to the database Saved code, and saved overhead of loading/storing large amounts of data

10 Persistent Prog. Languages (Cont.) In the past, database system developers proposed persistent versions of programming languages such as: Pascal C++, Java, Smalltalk Drawbacks of persistent programming languages Due to power of most programming languages, it is easy to make programming errors that damage the database. Complexity of languages makes automatic high-level optimization more difficult. Do not support declarative querying as well as relational databases

11 ORDB Features ORDB advocates evolutionary extensions to relational database so it is compatible with existing relational database technology while incorporating features in OO data model. Pioneering work started with Postgres in late 80 s (the commercial version of Postgres was known as Illustra, which was sold to Informix). Several ORDB systems: Oracle 8, Informix, UniSQL/X,...

12 ORDB Features (2) SQL-99 is the first SQL standard to support ORDB. The standard has not been fully implemented by commercial DBMSs. Oracle 8 and Informix have their own syntax for ORDB features currently.

13 Some ORDB Extensions allow composite attributes allow multi-valued attributes allow table attributes (nested tables) allow inheritance (not yet for Oracle) allow the creation of methods Support Object Identifiers (OIDs) Encapsulation is compromised!

14 Composite Attribute (1) Create an object type (user-defined type) SQL> create type name_type as object 2 (fname varchar(30), mi char(1), 3 lname varchar(30)); 4 / /* / is needed */ Now name_type can be used as any system provided types when defining an attribute. create table professors (id number(5), name name_type, office varchar(10));

15 Composite Attribute:Example id Name (fname, mi, lname) (Xiaofeng, null, Meng) (Philippe, L, Bonnet) office CS4 IS2

16 Composite Attribute (2) Create a tuple for the Professors table insert into professors values (12345, name_type( Xiaofeng, null, Meng ), IS4 ); name_type( ) serves as an object constructor. Query the table select * from professors; ID NAME(FNAME, MI, LNAME) OFFICE NAME_TYPE( Xiaofeng, NULL, Meng ) IS4

17 Composite Attribute (3) Dot notation for referring to composite attri More queries select p.name.lname from professors p; The alias p must be used. Dot notation (.) is used to qualify nested attributes and such expressions are called path expressions. Path expression can also be used in where clause. select p.office from professors p where p.name.lname = Meng ;

18 Composite Attribute (4) Create more complex types from existing ones. create type advisor_type as object (name name_type, rank varchar(20), phone# varchar(15)); Type dependency: type advisor_type depends on type name_type. name_type must be dropped first before advisor_type can be dropped. Example: drop type name_type; drop type advisor_type;

19 Composite Attribute (5) Create another table create table students (id number(5), name name_type, status varchar(10), advisor advisor_type); Insert a tuple insert into students values (23456, name_type( Mike, K, Allen ), Freshman, advisor_type(name_type( Lisa, S, Gauch ), Professor, ));

20 Composite Attribute (6) Create an object table create table advisors of advisor_type; Each advisor will be a row object and each advisor name will be a column object. All row objects are associated with a unique object identifier (oid) and this value can be accessed using ref( ) but the value is in hexadecimal format. select ref(a) from advisors a;

21 Composite Attribute (6):Example advisors oid 123A4B ((fname,mi, lname), rank, phone) (Lisa, S, Gray), A/Prof, (Xiaof, null, Meng), Prof,

22 Composite Attribute (7) One motivation for using object tables is that oid values of row objects can be used to navigate from an object in one table to a referenced object in another table. Such navigation may allow certain joins be avoided and as a result some queries can become more efficient.

23 Composite Attribute (8) Example: create type dept_type as object (dept# number(3), name varchar(30), location varchar(30)); create table departments of dept_type (primary key (dept#)); insert into departments values (123, CS, EB ); insert into departments values (234, Biology, S2 );

24 Composite Attribute (9) create type employee_type as object (emp# number(5), name name_type, age number(2), deptno number(5), dept ref dept_type); dept is an attribute whose values will be oids of department objects. create table employees of employee_type (primary key (emp#), foreign key deptno references departments (dept#), scope for (dept) is departments); dept will only reference department objects.

25 Composite Attribute (10) When load data to employees table, load all non-ref attributes first while let the ref attribute to have null values. Then use an update statement to load oids. insert into employees values (12345, name_type( Weiyi, null, Meng, 55, 123, null); insert into employees values (23456, name_type( Susan, J, Wilson, 45, 234, null); update employees e set dept = (select ref(d) from depatments d where d.dept# = e.deptno);

26 Composite Attribute (9):Example department oid Dept# name location O d1 123 CS EB O d2 234 Biology S2 employees oid Emp# name age deptno dept O e (Weiyi,null,Meng) null O e (Susan,J,Wilson) null

27 Composite Attribute (9):Example department oid Dept# name location O d1 123 CS EB O d2 234 Biology S2 employees oid Emp# name age deptno dept O e (Susan,J,Wilson) 123 (Weiyi,null,Meng) {O d1} O e {O d2 }

28 Composite Attribute (11) A query: Find the location of the department Prof. Meng works in. select e.dept.location from employees e where e.name.lname = Meng ; e.dept.location allows one to navigate from an employee object to a department object. The above is equivalent to the following query but more efficient: select d.location from departments d, employees e where e.name.lname = Meng and e.dept = ref(d);

29 Multi-valued Attribute (1) Two ways to create such an attribute in Oracle: Use table type (nested table) Use varray type (varying-length array) Example: create type advisee_type as table of name_type; create table faculty (id number(5) primary key, name name_type, rank varchar(30), advisees advisee_type ) nested table advisees store as advisees_table;

30 Multi-valued Attribute (2) Two tables are created in one statement: faculty, advisees_table advisees_table contains the names of all advisees of all faculty members; advisees_table can only be accessed via the faculty table. In general, a table may have multiple nested table attributes, each must have its own nested table clause separated by,.

31 id name rank advisees X Meng Prof fname mi lname Rui null Ding Daofeng null Luo P Bonnet Prof faculty id name X Meng P Bonne rank Prof Prof adv {o1,o2} {03} advisees oid fname o1 o2 Rui Daofe o3 mi null null lname Ding Luo

32 Multi-valued Attribute (3) Insert tuple insert into faculty values (12345, name_type( Xiaofeng, null, Meng ), Professor, advisee_type(name_type( Rui, null, Ding ), name_type( Daofeng, null, Luo ))); Query the table Find the ids of faculty members who have more than one advisee. select f.id from faculty f where 1 < (select count(*) from table(f.advisees));

33 Multi-valued Attribute (4) Function table( ) a value consisting of multiple elements under a nested table attribute into a table of these elements. table( ) cannot be used in the select clause. For a given faculty f with advisees, f.advisees is displayed as a single scalar value, i.e., count(f.advisees) yields 1 even when f has multiple advisees. Function table( ) unnests the single value into multiple elements.

34 Multi-valued Attribute (5) Example: Find the last names of all advisees of all faculty members. This query asks to retrieve a table of tables (note that each advisees value is a table). Oracle provides two special ways to do this: (1) Cartesian product select a.lname from faculty f, table(f.advisees) a (2) Nested cursor

35 Multi-valued Attribute (6) (2) Nested cursor select f.id, cursor(select a.lname advisee_names from table(f.advisees) a) advisee_tab from faculty f; * cursor(subquery) simulates the inner loop. Output: ID ADVISEE_TAB CURSOR STATEMENT: 2 CURSOR STATEMENT: 2 ADVISEE_NAMES Ding Luo

36 Multi-valued Attribute (7) varray type elements of varray are ordered; elements of varray are usually not stored in a separate table. xample: create type hobby_type as varray (3) of varchar(20); varray(n): at most n elements are allowed per varray value.

37 Multi-valued Attribute (8) create table members (id number(5) primary key, name varchar(20), hobbies hobby_type); insert into members values (12345, John, hobby_type( Golf, Tennis )); Even though varray elements are ordered, they cannot be accessed individually in SQL. Function table( ) can also be applied to a varray value.

38 Multi-valued Attribute (9) count and in can be applied to a varray value to recognize individual values. ind the number of hobbies of member select count(*) from members m, table(m.hobbies) where m.id = 12345; ind all members who like to play golf. select m.name from members m where Golf in (select * from table(m.hobbies));

39 Methods (1) In object-oriented model, methods defined on objects determine how the values of the objects can be accessed/modified. The feature that objects can be accessed or modified through their methods is known as encapsulation. Oracle supports the creation of methods for objects when object types are created. In Oracle, objects may be accessed/modified without the use of methods.

40 Methods (2) In Oracle, methods are created in two steps. Step 1: Define method signature: name, parameters, etc. Step 2: Create method body: PL/SQL code. xample: create type employees_type as object (id number(5), name varchar(20), birthdate date, salary number, member function age return int);

41 Methods (3) member function are the key words for defining a method. create or replace type body employees_type as member function age return int is begin return floor((sysdate self.birthdate)/365.25); end; end; self represents the object to which the method is applied.

42 Methods (4) create table employees_n of employees_type (primary key (id)); insert into employees_n values (12345, James, 15-DEC-60, 50000); insert into employees_n values (12345, Baker, 22-MAY-58, 60000); select e.name, e.age( ) from employees_n e where e.age( ) > 40; The alias e must be used in the above query.

43 Inheritance (1) Inheritance can be at the level of types, or at the level of tables create type person (name MyString, SSN integer) create type student (degree MyString, dept MyString) under person create type teacher (salary int, dept MyString ) under person

44 Inheritance (2) Multiple inheritance create type TA under student, teacher What s the problem here? create type TA under student with (dept as student-dept), teacher with (dept as teacher-dept)

45 Summary Complex types such as nested relations, are useful for modeling complex data in many application The relational model is extended beyond 1NF, to allow nonatomic types. Object-relational systems combine complex data based on an extended relational model with object-oriented concepts such as object identity and inheritance SQL have been extended to deal with complex types and object orientation.

46 Comparison of O-O and O-R Databases Summary of strengths of various database systems: Relational systems simple data types, powerful query languages, high protection. Persistent-programming-language-based OODBs complex data types, integration with programming language, high performance. Object-relational systems complex data types, powerful query languages, high protection. Note: Many real systems blur these boundaries E.g. persistent programming language built as a wrapper on a relational database offers first two benefits, but may have poor performance.

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49 Agreeeeeeeeee??