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Physical Database Design University of California, Berkeley School of Information Management and Systems SIMS 257: Database Management 9/26/2000 SIMS 257: Database Management Review • Normalization • Denormalization 9/26/2000 SIMS 257: Database Management Normalization No transitive dependency between nonkey attributes All determinants are candidate keys - Single multivalued dependency 9/26/2000 BoyceCodd and Higher SIMS 257: Database Management Functional dependencyof nonkey attributes on the primary key - Atomic values only Full Functional dependencyof nonkey attributes on the primary key Normalization • Normalization is performed to reduce or eliminate Insertion, Deletion or Update anomalies. • However, a completely normalized database may not be the most efficient or effective implementation. • “Denormalization” is sometimes used to improve efficiency. 9/26/2000 SIMS 257: Database Management Today • Physical Database Design • Access Methods • Indexes Based on McFadden Modern Database Management and Atre Database:Structured Techniques for Design, Performance and Management 9/26/2000 SIMS 257: Database Management Database Design Process Application 1 External Model Application 2 Application 3 Application 4 External Model External Model External Model Application 1 Conceptual requirements Application 2 Conceptual requirements Application 3 Conceptual requirements Conceptual Model Logical Model Internal Model Application 4 Conceptual requirements 9/26/2000 Physical Design SIMS 257: Database Management Physical Database Design • Many physical database design decisions are implicit in the technology adopted – Also, organizations may have standards or an “information architecture” that specifies operating systems, DBMS, and data access languages -- thus constraining the range of possible physical implementations. • We will be concerned with some of the possible physical implementation issues 9/26/2000 SIMS 257: Database Management Physical Database Design • The primary goal of physical database design is data processing efficiency • We will concentrate on choices often available to optimize performance of database services • Physical Database Design requires information gathered during earlier stages of the design process 9/26/2000 SIMS 257: Database Management Physical Design Information • Information needed for physical file and database design includes: – Normalized relations plus size estimates for them – Definitions of each attribute – Descriptions of where and when data are used • entered, retrieved, deleted, updated, and how often – Expectations and requirements for response time, and data security, backup, recovery, retention and integrity – Descriptions of the technologies used to implement the database 9/26/2000 SIMS 257: Database Management Physical Design Decisions • There are several critical decisions that will affect the integrity and performance of the system. – – – – – 9/26/2000 Storage Format Physical record composition Data arrangement Indexes Query optimization and performance tuning SIMS 257: Database Management Storage Format • Choosing the storage format of each field (attribute). The DBMS provides some set of data types that can be used for the physical storage of fields in the database • Data Type (format) is chosen to minimize storage space and maximize data integrity 9/26/2000 SIMS 257: Database Management Objectives of data type selection • • • • Minimize storage space Represent all possible values Improve data integrity Support all data manipulations • The correct data type should, in minimal space, represent every possible value (but eliminated illegal values) for the associated attribute and can support the required data manipulations (e.g. numerical or string operations) 9/26/2000 SIMS 257: Database Management Access Data Types • • • • • • • • • Numeric (1, 2, 4, 8 bytes, fixed or float) Text (255 max) Memo (64000 max) Date/Time (8 bytes) Currency (8 bytes, 15 digits + 4 digits decimal) Autonumber (4 bytes) Yes/No (1 bit) OLE (limited only by disk space) Hyperlinks (up to 64000 chars) 9/26/2000 SIMS 257: Database Management Access Numeric types • Byte – Stores numbers from 0 to 255 (no fractions). 1 byte • Integer – Stores numbers from –32,768 to 32,767 (no fractions) 2 bytes • Long Integer • Single (Default) – Stores numbers from –2,147,483,648 to 2,147,483,647 (no fractions). 4 bytes – Stores numbers from -3.402823E38 to –1.401298E–45 for negative values and from 1.401298E–45 to 3.402823E38 for positive values. 4 bytes • Double – Stores numbers from –1.79769313486231E308 to – 4.94065645841247E–324 for negative values and from 1.79769313486231E308 to 4.94065645841247E–324 for positive values. 15 8 bytes • Replication ID – Globally unique identifier (GUID) 9/26/2000 SIMS 257: Database Management N/A 16 bytes Controlling Data Integrity • • • • • Default values Range control Null value control Referential integrity Handling missing data 9/26/2000 SIMS 257: Database Management Designing Physical Records • A physical record is a group of fields stored in adjacent memory locations and retrieved together as a unit • Fixed Length and variable fields 9/26/2000 SIMS 257: Database Management Designing Physical Files/Internal Model • Overview • terminology • Access methods 9/26/2000 SIMS 257: Database Management Physical Design • Internal Model/Physical Model User request Interface 1 External Model DBMS Internal Model Access Methods Interface 2 Operating System Access Methods Interface 3 Data Base 9/26/2000 SIMS 257: Database Management Physical Design • Interface 1: User request to the DBMS. The user presents a query, the DBMS determines which physical DBs are needed to resolve the query • Interface 2: The DBMS uses an internal model access method to access the data stored in a logical database. • Interface 3: The internal model access methods and OS access methods access the physical records of the database. 9/26/2000 SIMS 257: Database Management Physical File Design • A Physical file is a portion of secondary storage (disk space) allocated for the purpose of storing physical records • Pointers - a field of data that can be used to locate a related field or record of data • Access Methods - An operating system algorithm for storing and locating data in secondary storage • Pages - The amount of data read or written in one disk input or output operation 9/26/2000 SIMS 257: Database Management Internal Model Access Methods • Many types of access methods: – – – – – – Physical Sequential Indexed Sequential Indexed Random Inverted Direct Hashed • Differences in – Access Efficiency – Storage Efficiency 9/26/2000 SIMS 257: Database Management Physical Sequential • Key values of the physical records are in logical sequence • Main use is for “dump” and “restore” • Access method may be used for storage as well as retrieval • Storage Efficiency is near 100% • Access Efficiency is poor (unless fixed size physical records) 9/26/2000 SIMS 257: Database Management Indexed Sequential • Key values of the physical records are in logical sequence • Access method may be used for storage and retrieval • Index of key values is maintained with entries for the highest key values per block(s) • Access Efficiency depends on the levels of index, storage allocated for index, number of database records, and amount of overflow • Storage Efficiency depends on size of index and volatility of database 9/26/2000 SIMS 257: Database Management Index Sequential Data File Actual Value 9/26/2000 Address Block Number Dumpling 1 Harty 2 Texaci 3 ... … Adams Becker Dumpling SIMS 257: Database Management Block 1 Getta Harty Block 2 Mobile Sunoci Texaci Block 3 Indexed Sequential: Two Levels Key Value Key Value 150 1 385 2 001 003 . . 150 Address 385 7 678 8 805 9 … Key Value 251 . . 385 Address 536 3 678 4 Key Value 9/26/2000 Address 455 480 . . 536 605 610 . . 678 Address 785 5 805 6 SIMS 257: Database Management 791 . . 805 705 710 . . 785 Indexed Random • Key values of the physical records are not necessarily in logical sequence • Index may be stored and accessed with Indexed Sequential Access Method • Index has an entry for every data base record. These are in ascending order. The index keys are in logical sequence. Database records are not necessarily in ascending sequence. • Access method may be used for storage and retrieval 9/26/2000 SIMS 257: Database Management Indexed Random Becker Harty Actual Value Address Block Number Adams 2 Becker 1 Dumpling 3 Getta 2 Harty 1 Adams Getta Dumpling 9/26/2000 SIMS 257: Database Management Btree F B || D || F| || P || Z| H || L || P| R || S || Z| Devils Aces Boilers Cars 9/26/2000 Flyers Hawkeyes Hoosiers Minors Panthers SIMS 257: Database Management Seminoles Inverted • Key values of the physical records are not necessarily in logical sequence • Access Method is better used for retrieval • An index for every field to be inverted may be built • Access efficiency depends on number of database records, levels of index, and storage allocated for index 9/26/2000 SIMS 257: Database Management Inverted CH 145 101, 103,104 Actual Value Address Block Number CH 145 1 CS 201 2 CS 623 3 PH 345 … CS 201 102 CS 623 105, 106 9/26/2000 SIMS 257: Database Management Student name Course Number Adams CH145 Becker cs201 Dumpling ch145 Getta ch145 Harty cs623 Mobile cs623 Direct • Key values of the physical records are not necessarily in logical sequence • There is a one-to-one correspondence between a record key and the physical address of the record • May be used for storage and retrieval • Access efficiency always 1 • Storage efficiency depends on density of keys • No duplicate keys permitted 9/26/2000 SIMS 257: Database Management Hashing • Key values of the physical records are not necessarily in logical sequence • Many key values may share the same physical address (block) • May be used for storage and retrieval • Access efficiency depends on distribution of keys, algorithm for key transformation and space allocated • Storage efficiency depends on distibution of keys and algorithm used for key transformation 9/26/2000 SIMS 257: Database Management Comparative Access Methods Factor Storage space Sequential retrieval on primary key Random Retr. Multiple Key Retr. Deleting records Sequential No wasted space Indexed Hashed No wasted space for data but extra space for index more space needed for addition and deletion of records after initial load Very fast Moderately Fast Impractical Moderately Fast Impractical Possible but needs Very fast with multiple indexes a full scan can create wasted OK if dynamic space OK if dynamic Adding records requires rewriting file Easy but requires Maintenance of Updating records usually requires indexes rewriting file 9/26/2000 SIMS 257: Database Management Very fast Not possible very easy very easy very easy