Introduction to Database Systems

Introduction to Database Systems

Overview of Database Systems Chpt 1 Instructor: Jianping Fan Database Management Systems Ramakrishnan & Gehrke Structural Components These layers must consider concurrency control and recovery Query Optimization and Execution Relational Operators Files and Access Methods Database Management Systems Buffer Management Disk Space Management

DB Ramakrishnan & Gehrke Structural Components Database Access Management Query Optimization and Execution Relational Operators Buffer Management Databased Indexing Management Files and Indexing Methods Data Storage Management Disk Space Management Database Management Systems Ramakrishnan & Gehrke Operational Components

User-System Interface Database Indexing Database Indexing Data Tables in Storage Disk Database Management Systems Ramakrishnan & Gehrke Query Optimizers What is Database? User point of view: service computing Database: tables & indexing & storage & UCI Database Indexing Database Management Systems Database Indexing

Data Storage Disk Ramakrishnan & Gehrke Database Indexing What Is a DBMS? Developer point of view: software package A very large, integrated collection of data. Models real-world enterprise. Database Components Entities (e.g., students, courses) Relationships (e.g., Madonna is taking CS564) A Database Management System (DBMS) is a software package designed to store and manage databases. Database Management Systems

Ramakrishnan & Gehrke What Is a DBMS? DBMS programmers Users & Database Management Systems DBMS Vendors Ramakrishnan & Gehrke Why not Use a File Systems? Need big memory for large data set. Long time for data access over large data set. Cannot support concurrent access by many users.

Not easy for system recovery. Bad for access controlshow whole data to users.Turing award for database researchers: 3 Database Management Systems Ramakrishnan & Gehrke Why Use a DBMS? Compare with file systems? Data independence by representing the data in high-level abstract---you just know what you are permitted Efficient access via database indexing---very fast Reduced application development time without changing the data---easy for package design and add

Data integrity (constraints) and securityeasy for access control. Uniform data administrationdata understanding and management, easy for data sharing Concurrent access, recovery from crashes --serve many users Database Management Systems Ramakrishnan & Gehrke Why Study Databases?? ? Release people from computation to information at the low end: scramble to webspace at the high end: scientific applications Release people from data collections ---Digital libraries, interactive video, Human Genome project, EOS project ... need for DBMS exploding

DBMS encompasses most of CS OS, languages, theory, AI, multimedia, logic Database Management Systems Ramakrishnan & Gehrke 1 Data Models: E-R Models A data model is a collection of concepts for describing data. A schema is a description of a particular collection of data, using the a given data model. The relational model of data is the most widely used model today.

Main concept: relation, basically a table with rows and columns. Every relation has a schema, which describes the columns, or fields. Database Management Systems Ramakrishnan & Gehrke 1 Data Models--- how to build database for this? Sid name login age GPA

53666 Jones [email protected] 18 3.4 53688 Smith [email protected] 18 3.2 53650 Tom [email protected]

19 3.3 53678 Jim [email protected] Database Management Systems Ramakrishnan & Gehrke 20 3.5 1 Levels of Abstraction Many views, single conceptual (logical) schema and physical

schema. Views describe how users see the data. View 1 View 2 View 3 Conceptual Schema Conceptual schema defines logical structurehow to describe the stored data Physical schema describes the files and indexes used how to store the concept scheme?. Physical Schema Schemas are defined using DDL; data is modified/queried using DML. Database Management Systems Ramakrishnan & Gehrke 1

Example: University Database Conceptual schema: Students(sid: string, name: string, login: string, age: integer, gpa:real) Courses(cid: string, cname:string, credits:integer) Enrolled(sid:string, cid:string, grade:string) Physical schema: Relations stored as unordered files. Index on first column of Students. External Schema (View): Course_info(cid:string,enrollment:integer) Database Management Systems Ramakrishnan & Gehrke 1

Data Independence Applications define how data is structured and stored. Logical data independence: Protection from changes in logical structure of data. Physical data independence: Protection from changes in physical structure of data. One of the most important benefits of using a DBMS! Database Management Systems Ramakrishnan & Gehrke 1

Concurrency Control Concurrent execution of user programs is essential for good DBMS performance. Many people may access the same data at the same time. DBMS ensures that many users can pretend they are using a single-user system. ---fast and safe Database Management Systems Ramakrishnan & Gehrke 1 Transaction: An Execution of a DB Program

Transaction is an atomic sequence of database actions (reads/writes). transaction == data access execution Each transaction, executed completely, must leave the DB in a consistent state. DBMS permits concurrent execution of transactions. Database Management Systems Ramakrishnan & Gehrke 1 Scheduling Concurrent Transactions DBMS ensures that execution of {T1, ... , Tn} is equivalent to some serial execution T1 ... Tn. Before reading/writing an object, a transaction requests a lock on the object, and waits till the

DBMS gives it the lock. All locks are released at the end of the transaction. Idea: If an action of Ti (say, writing X) affects Tj (which perhaps reads X), one of them, say Ti, will obtain the lock on X first and Tj is forced to wait until Ti completes; this effectively orders the transactions.---first come in, first service. Database Management Systems Ramakrishnan & Gehrke 1 Ensuring Atomicity DBMS ensures atomicity (all-or-nothing property) even if system crashes in the middle of a Xact. Idea: Keep a log (history) of all actions carried out by the DBMS while executing a set of Xacts: Before a change is made to the database, the corresponding log entry is forced to a safe location. (WAL protocol; OS support for this is often inadequate.)

After a crash, the effects of partially executed transactions are undone using the log. (Thanks to WAL, if log entry wasnt saved before the crash, corresponding change was not applied to database!) Database Management Systems Ramakrishnan & Gehrke 1 The Log The following actions are recorded in the log: Ti writes an object: the old value and the new value. Log record must go to disk before the changed page! Ti commits/aborts: a log record indicating this action.

Log records chained together by Xact id, so its easy to undo a specific Xact (e.g., to resolve a deadlock). Log is often duplexed and archived on stable storage. All log related activities (and in fact, all concurrency control related activities such as lock/unlock, dealing with deadlocks etc.) are handled transparently by the DBMS. Database Management Systems Ramakrishnan & Gehrke 2 Databases make these folks happy ... End users and DBMS vendors DB application programmers E.g. smart webmasters

Database administrator (DBA) Designs logical /physical schemas Handles security and authorization Data availability, crash recovery Database tuning as needs evolve Must understand how a DBMS works! Database Management Systems Ramakrishnan & Gehrke 2 Structure of a DBMS

These layers must consider concurrency control and recovery A typical DBMS has a Query Optimization layered architecture. and Execution The figure does not Relational Operators show the concurrency control and recovery Files and Access Methods components. Buffer Management This is one of several possible architectures; Disk Space Management each system has its own variations. Database Management Systems DB

Ramakrishnan & Gehrke 2 Steps for DBMS Design a. User requirement analysis: indexing & storage a. Data analysis: how to manage the data under the requirement? a. Database design: logical design---select suitable data model physical design---select suitable indexing d. Access management design: how to serve large-scale users? e. Efficiency concern: how to manage large-scale data set? Database Management Systems Ramakrishnan & Gehrke

2 Summary DBMS used to maintain, query large datasets. Benefits include recovery from system crashes, concurrent access, quick application development, data integrity and security. Levels of abstraction give data independence. A DBMS typically has a layered architecture. DBAs hold responsible jobs and are well-paid! DBMS R&D is one of the broadest, most exciting areas in CS. Database Management Systems Ramakrishnan & Gehrke

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