Ch1- Introduction to dbms

Ch1- Introduction to dbms

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  CChhaapptteerr 11:: IInnttrroodduuccttiioonn
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  Database Management System (DBMS) Definition: DBMS contains information about a particular enterprise, like- Collection of interrelated data Set of programs to access the data An environment that is both convenient and efficient to use Databases can be very large. Database Applications: Banking: transactions Airlines: reservations, schedules Universities: registration, grades Sales: customers, products, purchases Online retailers: order tracking, customized recommendations Manufacturing: production, inventory, orders, supply chain Human resources: employee records, salaries, tax deductions Example, University database.
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  Drawbacks of usingfile systems to store data Data redundancy and inconsistency  Multiple file formats, duplication of information in different files Difficulty in accessing data  Need to write a new program to carry out each new task Data isolation — multiple files and formats Integrity problems  Hard to add Integrity constraints (e.g., account balance 0) or change existing ones Atomicity of updates  Failures may leave database in an inconsistent state with partial updates carried out  Example: Transfer of funds from one account to another should either complete or not happen at all
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  Drawbacks of usingfile systems to store data (Cont.) Concurrent access by multiple users  Concurrent access needed for performance  Uncontrolled concurrent accesses can lead to inconsistencies – Example: Two people reading a balance (say 100) and updating it by withdrawing money (say 50 each) at the same time Security problems  Hard to provide user access to some, but not all, data Database systems offer solutions to all the above problems
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  Levels of Abstraction Physical level: lowest level of abstraction. Describes how a record (e.g., customer) is stored. Very much complex. Logical level: next-higher level. Describes data stored in database, and the relationships among the data. type instructor = record ID : string; name : string; dept_name : string; salary : integer; end; View level: application programs hide details of data types. Views can also hide information (such as an employee’s salary) for security purposes.
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  View of Data An architecture for a database system
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  Instances and Schemas Similar to types and variables in programming languages SScchheemmaa – the logical structure of the database Example: The database consists of information about a set of customers and accounts and the relationship between them Analogous to type information of a variable in a program Physical schema: database design at the physical level Logical schema: database design at the logical level Instance – the actual content of the database at a particular point in time Analogous to the value of a variable Physical Data Independence – the ability to modify the physical schema without changing the logical schema Applications depend on the logical schema In general, the interfaces between the various levels and components should be well defined so that changes in some parts do not seriously influence others.
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  Data Models Definition: A collection of tools for describing Data Data relationships Data semantics Data constraints Types of data models: Relational model Entity-Relationship data model (mainly for database design) Object-based data models (Object-oriented and Object-relational) Semistructured data model (XML) Other older models: Network model Hierarchical model
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  Relational Model Example of tabular data in the relational model Columns Rows
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  A Sample RelationalDatabase
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  Data Manipulation Language (DML) Language for accessing and manipulating the data organized by the appropriate data model DML also known as query language Two classes of languages Procedural – user specifies what data is required and how to get those data Declarative (nonprocedural) – user specifies what data is required without specifying how to get those data SQL is the most widely used query language
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  Data Definition Language(DDL) Specification notation for defining the database schema Example: create table instructor ( ID char(5), name varchar(20), dept_name varchar(20), salary numeric(8,2)) DDL compiler generates a set of table templates stored in a data dictionary Data dictionary: contains metadata (i.e., data about data) of a database. It has- Database schema Integrity constraints  Primary key (ID uniquely identifies instructors)  Referential integrity (references constraint in SQL) Authorization
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  The Entity-Relationship Model Models an enterprise as a collection of entities and relationships Entity: a “thing” or “object” in the enterprise that is distinguishable from other objects  Described by a set of attributes Relationship: an association among several entities Represented diagrammatically by an entity-relationship diagram: What happened to dept_name of instructor and student?
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  Storage Management Storagemanager is a program module that provides the interface between the low-level data stored in the database and the application programs and queries submitted to the system. The storage manager is responsible to the following tasks: Interaction with the file manager Efficient storing, retrieving and updating of data Provides: Storage access File organization Indexing and hashing
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  Query Processing 1.Parsing and translation 2. Optimization 3. Evaluation
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  Query Processing (Cont.) Alternative ways of evaluating a given query Equivalent expressions Different algorithms for each operation Cost difference between a good and a bad way of evaluating a query can be enormous Need to estimate the cost of operations Depends critically on statistical information about relations which the database must maintain Need to estimate statistics for intermediate results to compute cost of complex expressions
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  Transaction Management Atransaction is a collection of operations that performs a single logical function in a database application Transaction-management component ensures that the database remains in a consistent (correct) state despite system failures (e.g., power failures and operating system crashes) and transaction failures. Concurrency-control manager controls the interaction among the concurrent transactions, to ensure the consistency of the database.
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  Database Architecture Thearchitecture of a database systems is greatly influenced by the underlying computer system on which the database is running: Centralized Client-server Parallel (multi-processor) Distributed
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  Thank you