Chapter Five Physical Database Design

Transcription

1 Chapter Five Physical Database Design 1

2 Objectives Understand Purpose of physical database design Describe the physical database design process Choose storage formats for attributes Describe indexes and their appropriate use 2

3 Logical vs Physical Database Design Logical database design is independent of implementation details, such as functionality of target DBMS. Logical database design concerned with the what, physical database design is concerned with the how. Physical Design is description of the implementation of the database on secondary storage using the target DBMS 3

4 Physical Database Design Process of producing a description of implementation of the database on secondary storage. The main tasks including: field specification, choosing indexes and file organization, and to refine the conceptual and external schemas (if necessary) to meet performance goals. 4

5 Cont The primary goal of physical database design is data processing efficiency (Time and Storage impact) We will concentrate on choices often available to optimize performance of database services Many physical database design decisions are implicit in the technology adopted 5

6 Cont Design decision about 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 optimize storage space and maximize data integrity 6

7 Cont It is also called Designing Fields Field Smallest unit of named application data recognized by system software Attributes from relations will be represented as fields In addition to Choosing data type you may also consider Coding (optional) Controlling data integrity Data Type A coding scheme recognized by system software for representing organizational data 7 7

8 Cont Choosing Data Types CHAR fixed-length character VARCHAR variable-length character (memo) LONG large number NUMBER positive/negative number INEGER positive/negative whole number DATE actual date BLOB binary large object (good for graphics, sound clips, etc.) 8

9 Cont Objectives of choosing data types Minimize storage space Represent all possible values of the field Improve data integrity of the field Support all data manipulations desired on the field 9

10 Example storage format (field design) for the Branch table 10 10

11 Cont Choose indexes Index in a database is similar to an index in a book Primary indexes is automatic by the primary key Clustering index- by non key attribute Guidelines for Choosing Indexes Specify a unique index for the primary key of each table. Specify an index for foreign keys. Specify an index for nonkey fields that are referenced in qualification, sorting and grouping commands for the purpose of retrieving data

12 Rules for Using Indexes 1. Use on larger tables 2. Index the primary key of each table 3. Index search fields (fields frequently in WHERE clause) 4. Fields in SQL ORDER BY and GROUP BY commands 5. When there are >100 values but not when there are <30 values 12

13 Rules for Using Indexes (cont.) 6. Avoid use of indexes for fields with long values; perhaps compress values first 7. DBMS may have limit on number of indexes per table and number of bytes per indexed field(s) 8. Null values will not be referenced from an index 9. Use indexes heavily for non-volatile databases; limit the use of indexes for volatile databases Why? Because modifications (e.g. inserts, deletes) require updates to occur in index files 13

14 14 End Of Chapter Five