Physical Database Design University of California Berkeley School
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 Boyce. Codd 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 Mc. Fadden 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 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. – Storage Format – Physical record composition – Data arrangement – Indexes – Query optimization and performance tuning 9/26/2000 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. 402823 E 38 to – 1. 401298 E– 45 for negative values and from 1. 401298 E– 45 to 3. 402823 E 38 for positive values. 4 bytes • Double – Stores numbers from – 1. 79769313486231 E 308 to – 4. 94065645841247 E– 324 for negative values and from 1. 79769313486231 E 308 to 4. 94065645841247 E– 324 for positive values. 15 8 bytes • Replication ID 9/26/2000 – Globally unique identifier (GUID) 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 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 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. . 805 710. . 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 CH 145 Becker cs 201 Dumpling ch 145 Getta ch 145 Harty cs 623 Mobile cs 623
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 Possible but needs Very fast with multiple indexes a full scan create wasted OK if dynamic space Adding records requires rewriting OK if dynamic 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
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