CENG 351 Introduction to Data Management and File Structures

1. CENG 351 Introduction to Data Management and File Structures Nihan Kesim Çiçekli Department of Computer Engineering METU CENG 351

2. CENG 351-Section 2 • Instructor: Nihan Kesim Çiçekli • Office: A308 • Email: nihan@ceng.metu.edu.tr • Lecture Hours: Tue. 15:40; Thu. 11:40,12:40 (BMB3) • Course Web page: http://cow.ceng.metu.edu.tr • Teaching Assistants: Hande Çelikkanat, Cüneyt Mertayak, Çağatay Çallı CENG 351

3. References • Betty Salzberg, File Structures: An Analytic Approach, Prentice Hall, 1988. • Raghu Ramakrishnan, Database Management Systems (3rd. ed.), McGraw Hill, 2003. • Michael J. Folk, Bill Zoellick and Greg Riccardi, File Structures, An object oriented approach with C++, Addison-Wesley, 1998. • R. Elmasri, S.B. Navathe, Fundamentals of Database Systems, 4th edition, Addison-Wesley, 2004. CENG 351

4. Course Outline • Introduction: Secondary storage devices • Fundamental File Structure Concepts: Sequential Files • External Sorting • Indexed Sequential Files (B-trees) • Direct access (Hashing) • Introduction to Database Systems:E/R modeling, relational model, • Query languages: Relational algebra, relational calculus, SQL • Query Evaluation CENG 351

5. Grading 3 written HW, 3 programming assignments 30% Midterm Exam 1 20% Midterm Exam 2 20% Final Exam 30% Tentative Exam Dates: Midterm Exam 1: Nov. 6, 2008 Midterm Exam 2: Dec. 18, 2007 CENG 351

6. Grading Policies • Policy on missed midterm: • no make-up exam • Lateness policy: • Late assignments are penalized up to 10% per day. • All assignmentsand programs are to be your own work. No group projects or assignments are allowed. CENG 351

7. Introduction to File management CENG 351

8. Motivation • Most computers are used for data processing (over $100 billion/year). A big growth area in the “information age” • This course covers data processing from a computer science perspective: • Storage of data • Organization of data • Access to data • Processing of data CENG 351

9. Data Structures vs File Structures • Both involve: • Representation of Data + • Operations for accessing data • Difference: • Data structures: deal with data in main memory • File structures: deal with data in secondary storage CENG 351

10. Where do File Structures fit in Computer Science? Application DBMS File system Operating System Hardware CENG 351

11. Computer Architecture data is manipulated here - Semiconductors - Fast, expensive, volatile, small Main Memory (RAM) data transfer Secondary Storage - disks, tape - Slow,cheap, stable, large data is stored here CENG 351

12. Advantages • Main memory is fast • Secondary storage is big (because it is cheap) • Secondary storage is stable (non-volatile) i.e. data is not lost during power failures Disadvantages • Main memory is small. Many databases are too large to fit in main memory (MM). • Main memory is volatile, i.e. data is lost during power failures. • Secondary storage is slow (10,000 times slower than MM) CENG 351

13. How fast is main memory? • Typical time for getting info from: Main memory: ~12 nanosec = 120 x 10-9 sec Magnetic disks: ~30 milisec = 30 x 10-3 sec • An analogy keeping same time proportion as above: Looking at the index of a book : 20 sec versus Going to the library: 58 days CENG 351

14. Normal Arrangement • Secondary storage (SS) provides reliable, long-term storage for large volumes of data • At any given time, we are usually interested in only a small portion of the data • This data is loaded temporarily into main memory, where it can be rapidly manipulated and processed. • As our interests shift, data is transferred automatically between MM and SS, so the data we are focused on is always in MM. CENG 351

15. Goal of the file structures • Minimize the number of trips to the disk in order to get desired information • Grouping related information so that we are likely to get everything we need with only one trip to the disk. CENG 351

16. Physical Files and Logical Files • physical file: a collection of bytes stored on a disk or tape • logical file: a "channel" (like a telephone line) that connects the program to a physical file • The program (application) sends (or receives) bytes to (from) a file through the logical file. The program knows nothing about where the bytes go (came from). • The operating system is responsible for associating a logical file in a program to a physical file in disk or tape. Writing to or reading from a file in a program is done through the operating system. CENG 351

17. Files • The physical file has a name, for instance myfile.txt • The logical file has a logical name (a varibale) inside the program. • In C : FILE * outfile; • In C++: fstream outfile; CENG 351

18. Basic File Processing Operations • Opening • Closing • Reading • Writing • Seeking CENG 351

19. Opening Files • Opening Files: • links a logical file to a physical file. • In C: FILE * outfile; outfile = fopen(“myfile.txt”, “w”); • In C++: fstream outfile; outfile.open(“myfile.txt”, ios::out); CENG 351

20. Closing Files • Cuts the link between the physical and logical files. • After closing a file, the logical name is free to be associated to another physical file. • Closing a file used for output guarantees everything has been written to the physical file. (When the file is closed the leftover from the buffer is flushed to the file.) • In C : fclose(outfile); • In C++ : outfile.close(); CENG 351

21. Reading • Read data from a file and place it in a variable inside the program. • In C: char c; FILE * infile; infile = fopen(“myfile.txt”,”r”); fread(&c, 1, 1, infile); • In C++: char c; fstream infile; infile.open(“myfile.txt”,ios::in); infile >> c; CENG 351

22. Writing • Write data from a variable inside the program into the file. • In C: char c; FILE * outfile; outfile = fopen(“mynew.txt”,”w”); fwrite(&c, 1, 1, outfile); • In C++: char c; fstream outfile; outfile.open(“mynew.txt”,ios::out); outfile << c; CENG 351

23. Seeking • Used for direct access; an item can be accessed by specifying its position in the file. • In C: fseek(infile,0, 0); // moves to the beginning fseek(infile, 0, 2); // moves to the end fseek(infile,-10, 1); //moves 10 bytes from //current position • In C++: infile.seekg(0,ios::beg); infile.seekg(0,ios::end); infile.seekg(-10,ios::cur); CENG 351

24. File Systems • Data is not scattered hither and thither on disk. • Instead, it is organized into files. • Files are organized into records. • Records are organized into fields. CENG 351

25. Example • A student file may be a collection of student records, one record for each student • Each student record may have several fields, such as • Name • Address • Student number • Gender • Age • GPA • Typically, each record in a file has the same fields. CENG 351

26. Properties of Files • Persistance: Data written into a file persists after the program stops, so the data can be used later. • Sharability: Data stored in files can be shared by many programs and users simultaneously. • Size: Data files can be very large. Typically, they cannot fit into MM. CENG 351