ICOM 5016 Introduction to Database Systems Lecture 13
ICOM 5016 – Introduction to Database Systems Lecture 13 - File Structures Dr. Bienvenido Vélez Electrical and Computer Engineering Department Slides by Dr. Manuel Rodríguez
Readings • Read – New Book: Chapter 13 ICOM 5016 Dr. Manuel Rodriguez Martinez 2
Relational DBMS Architecture Client API Client Query Parser Query Optimizer Relational Operators Execution Engine File and Access Methods Buffer Management Concurrency and Recovery Disk Space Management DB ICOM 5016 Dr. Manuel Rodriguez Martinez 3
Disk Space Managemet • Disk Space Manager – DBMS module in charge of managing the disk space used to store relations – Duties • • Allocate space Write data Read data De-allocate space • Disk Space Manager supplies a stream of data pages. – Minimal unit of I/O – Often the size of a block (sector, several sectors, or more) ICOM 5016 Dr. Manuel Rodriguez Martinez 4
Relational DBMS Architecture Client API Client Query Parser Query Optimizer Relational Operators Execution Engine File and Access Methods Buffer Management Concurrency and Recovery Disk Space Management DB ICOM 5016 Dr. Manuel Rodriguez Martinez 5
Buffer Management • Buffer Manager supplies a stream of memory data pages. – Often the size of a block (sector, several sectors, or more) – Must provide in-memory access to more pages than physically fit in memory – Must implement a page replacement policy – Implements a cache of disk blocks ICOM 5016 Dr. Manuel Rodriguez Martinez 6
Relational DBMS Architecture Client API Client Query Parser Query Optimizer Relational Operators Execution Engine File and Access Methods Buffer Management Concurrency and Recovery Disk Space Management DB ICOM 5016 Dr. Manuel Rodriguez Martinez 7
Disk Page • Disk page is simply an array of bytes • We impose the logic of an array of records! 123 Bob NY $1200 2178 Jil LA $9202 8273 Ned FL $2902 723 Al PR $300 Records Disk Page Reading a Disk Page should be one I/O ICOM 5016 Dr. Manuel Rodriguez Martinez 8
Storage arrangement options • Suppose we need to create 10 GB of space to store a database. Each page is 4 KB is size. – How to organize the disk to accomplish this. • Option #1: Cooked File System – User the file system provide by OS – Create a file “mydb. dat” – Write to this file N pages of size 4 KB • N must be enough to reach the size of 10 GB • Page are full of bytes with zeros. – Have a has table somewhere to store the information about this file “mydb. dat”. – Now you can start writing pages with actual data. ICOM 5016 Dr. Manuel Rodriguez Martinez 9
Option #2: Raw Disk Partition • Don’t use the file provided by the DBMS • Instead create a parition on the disk, but don’t format it with OS formats (e. g. FAT, FAT 32, NTFS, LINUX) • Make your own file system on the disk – Create a directory of pages – Need to implement all operation such as read, write, check, etc. – Need to implement you own files… – Faster and more efficient that OS files, but more complex. – Provided by more advanced (and expensive) DBMS’s ICOM 5016 Dr. Manuel Rodriguez Martinez 10
Alternative DBMS Views of the Storage System • Single File – – Each database is stored as a single file Example: SQLite Pro: Easy to store and exchange databases Cons: Concurrent/Transactional access very hard • File System – – Each database is stored as multiple files Example: My. SQL Pro: Concurrent/Transactional access good, DBMS portable and simpler Cons: DBMS must rely on OS for performance • Block System (Raw Disk) – – The DBMS implements its own storage system on raw disk partition Example: Oracle 10 g Pro: RDBMS can achieve maximal performance Cons: RDBMS complex/expensive
File and Access Methods Layer • Buffer Manager provides a stream of pages • But higher layers of DBMS need to see a stream of records • A DBMS file layers provides this abstraction – File is a collection of records that belong to a relation R. • For example: Relation students might be stored in DBMS internal file students. dat. This is internal to DBMS database!!! – File is made out of pages, and records are taken from pages • File and Access Methods Layer implements various types of files to access the records – Access method – mechanism by which the records are extracted from the DBMS ICOM 5016 Dr. Manuel Rodriguez Martinez 12
File Types • Heap File - Unordered collection of records – Records within a page a not ordered – Pages are not ordered – Simple to use and implement • Sorted File – sorted collection or records – Within a page, records are ordered – Pages are ordered based on record contents – Efficient access to data, but expensive to maintain • Index File – combines storage + data structure for fast access and lookups – Index entries – store value of attributes as search keys – Data entries – hold the data in the index file ICOM 5016 Dr. Manuel Rodriguez Martinez 13
Heap File 123 Bob NY $102 8387 Ned SJ $73 121 Jil NY $5595 81982 Tim MIA $4000 2381 Bill LA $500 4882 Al SF $52303 Ned NY 9403 Page 0 Page 1 $3333 Page 2 1237 ICOM 5016 Pat WI $30 Dr. Manuel Rodriguez Martinez 14
Sorted File 121 Jil NY $5595 123 Bob NY $102 1237 Pat WI $30 2381 Bill LA $500 4882 Al SF $52303 8387 Ned SJ $73 9403 Ned NY $3333 81982 Tim MIA $4000 ICOM 5016 Dr. Manuel Rodriguez Martinez Page 0 Page 1 Page 2 15
Index File 121 Jil NY $5595 123 Bob NY $102 1237 Pat WI $30 100 2000 9000 Index entry ICOM 5016 2381 Bill LA $500 4882 Al SF $52303 8387 Ned SJ $73 9403 Ned NY $3333 81982 Tim MIA $4000 Dr. Manuel Rodriguez Martinez Data entries 16
Index files structure • Index entries – Store search keys – Search key – a set of attributes in a tuple can be used to guide a search • Ex. Student id – Search key do not necessarily have to be candidate keys • For example: gpa can be a search key on relation: Students(sid, name, login, age, gpa) • Data entries – Store the data records in the index file – Data record can have • Actual tuples for the table on which index is defined • Record identifier for tuples that match a given search key ICOM 5016 Dr. Manuel Rodriguez Martinez 17
Issues with Index files • Index files for a relation R can occur in three forms: – Data entries store the actual data for relation R. • Index file provides both indexing and storage. – Data entries store pairs <k, rid>: • k – value for a search key. • rid – rid of record having search key value k. • Actual data record is stored somewhere else, perhaps on a heap file or another index file. – Data entries store pairs <k, rid-list> • K – value for a search key • Rid-list – list of rid for all records having search key value k • Actual data record is stored somewhere else, perhaps on a heap file or another index file. ICOM 5016 Dr. Manuel Rodriguez Martinez 18
Operations on files • Allocate file • Scan operations – Grab each records one after one • Can be used to step through all records • Insert record – Adds a new record to the file • Each record as a unique identifier called the record id (rid) • • Update record Find record with a given rid Delete record with a given rid De-allocate file ICOM 5016 Dr. Manuel Rodriguez Martinez 19
Implementing Heap Files • Heap file links a collection of pages for a given relation R. • Heap files are built on top of Buffer Manager. • Each page has a page id – Often, we need to know the page size (e. g. 4 KB) • All pages for a given file have the same size. – Page id and page size can be used to compute an offset in a cooked file where the page is located. – In raw disk partition, page id should enable DBMS to find block in disk where the page is located. ICOM 5016 Dr. Manuel Rodriguez Martinez 20
Linked Implementation of Heap Files Data Page Linked List of pages With free space Header Page Data Page Linked List of full pages ICOM 5016 Dr. Manuel Rodriguez Martinez 21
Linked List of pages • Each page has: – records – pointer to next page – pointer to previous page • Pointer here means the integer with the page id of the next page. • Header has two pointers – First page in the list of pages with free space – First page in the list of full pages • Tradeoffs – Easy to use, good for fixed sized records – Complex to find space for variable length records • need to iterate over list with space ICOM 5016 Dr. Manuel Rodriguez Martinez 22
Directory of pages Data Page 1 header Data Page 2 Data Page 3. . . Data Page N ICOM 5016 Dr. Manuel Rodriguez Martinez 23
Directory of pages • Linked list of directory pages • Directory page has – Pointer to a given page – Bit indicating if page is full or not – Alternatively, have amount of space that is available • More complex to implement • Makes it easier to find page with enough room to store a new record ICOM 5016 Dr. Manuel Rodriguez Martinez 24
Page formats • Each page holds – records – Optional metadata for finding records within the page • Page can be visualized as a collection of slots where records can be placed • Each record has a record id in the form: – <page_id, slot number> – page_id – id of the page where the record is located. – slot number – slot where the record is located. ICOM 5016 Dr. Manuel Rodriguez Martinez 25
Packed Fixed-Length Record Slot 1 Slot 2 Slot 3. . . number of records Slot N Free Space N Page header ICOM 5016 Dr. Manuel Rodriguez Martinez 26
Unpacked Fixed-Length Record Slot 1 Slot 2 Slot 3. . . number of slots Free Space Slot N 1 0 3 2 Slot bit vector ICOM 5016 1 1 N Page header Dr. Manuel Rodriguez Martinez 27
Variable-Length records Page i 12 bytes rid=(i, N) rid=(1, N) rid=(2, N) Free Space 12 N ICOM 5016 … … 15 24 2 1 Dr. Manuel Rodriguez Martinez N entries 28
Fixed-Length records for variablelength attributes • Size of each record is determined by maximum size of the data type in each column Size in bytes F 1 F 2 F 3 F 4 2 6 2 4 Offset of F 1: 0 Offset of F 2: 2 Offset of F 3: 8 Offset of F 4: 10 ICOM 5016 Need to understand the schema and sizes to find a given column Dr. Manuel Rodriguez Martinez 29
Variable-length records and attributes • Either 1. use a special symbol to separate fields 2. use a header to indicate offset of each field Option 1 Option 2 F 1 $ F 2 $ F 3 F 1 $ F 2 F 4 F 3 F 4 Option 1 has the problem of determining a good $ Option 2 handles NULL easily ICOM 5016 Dr. Manuel Rodriguez Martinez 30
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