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Overview of Database Systems Chpt 1

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

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Overview of Database Systems Chpt 1

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  1. Overview of Database SystemsChpt 1 Instructor: Jianping Fan Ramakrishnan & Gehrke

  2. Query Optimization and Execution Relational Operators Files and Access Methods Buffer Management Disk Space Management DB These layers must consider concurrency control and recovery Structural Components Ramakrishnan & Gehrke

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

  4. Operational Components User-System Interface Database Indexing Database Indexing Query Optimizers Data Tables in Storage Disk Ramakrishnan & Gehrke

  5. What is Database? User point of view: service computing • Database: tables & indexing & storage & UCI Database Indexing Database Indexing Database Indexing Data Storage Disk Ramakrishnan & Gehrke

  6. 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. Ramakrishnan & Gehrke

  7. What Is a DBMS? DBMS programmers Users & DBMS Vendors Ramakrishnan & Gehrke

  8. 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 control—show whole data to users. Turing award for database researchers: 3 Ramakrishnan & Gehrke

  9. 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 security—easy for access control. • Uniform data administration—data understanding and management, easy for data sharing • Concurrent access, recovery from crashes --serve many users Ramakrishnan & Gehrke

  10. ? 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, “A”I, multimedia, logic Ramakrishnan & Gehrke

  11. Data Models: E-R Models • A data modelis a collection of concepts for describing data. • Aschemais a description of a particular collection of data, using the a given data model. • The relational model of datais 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. Ramakrishnan & Gehrke

  12. Data Models--- how to build database for this? Sid name login age GPA 53666 Jones jones@cs 18 3.4 53688 Smith smith@cs 18 3.2 53650 Tom tom@cs 19 3.3 53678 Jim jim@cs 20 3.5 Ramakrishnan & Gehrke

  13. Levels of Abstraction View 1 View 2 View 3 • Many views, single conceptual (logical) schemaand physical schema. • Views describe how users see the data. • Conceptual schema defines logical structure—how to describe the stored data • Physical schema describes the files and indexes used—how to store the concept scheme?. Conceptual Schema Physical Schema • Schemas are defined using DDL; data is modified/queried using DML. Ramakrishnan & Gehrke

  14. 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) Ramakrishnan & Gehrke

  15. 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! Ramakrishnan & Gehrke

  16. 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 Ramakrishnan & Gehrke

  17. Transaction: An Execution of a DB Program • Transaction is an atomicsequence 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. Ramakrishnan & Gehrke

  18. 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. Ramakrishnan & Gehrke

  19. 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 wasn’t saved before the crash, corresponding change was not applied to database!) Ramakrishnan & Gehrke

  20. 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 beforethe changed page! • Ti commits/aborts: a log record indicating this action. • Log records chained together by Xact id, so it’s 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. Ramakrishnan & Gehrke

  21. 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! Ramakrishnan & Gehrke

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

  23. Steps for DBMS Design • User requirement analysis: indexing & storage • Data analysis: how to manage the data under the requirement? • 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? Ramakrishnan & Gehrke

  24. 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. Ramakrishnan & Gehrke

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