Data base
- 3. DENORMALIZATION • Transforming normalized relations into unnormalized physical record specifications • – The process of splitting or combining normalized relations into physical tables based on affinity of use of rows and fields • Benefits: • Can improve performance (speed) be reducing number of table lookups (i.e reduce number of necessary join queries) • Costs (due to data duplication) • Wasted storage space • Data integrity/consistency threats
- 4. DENORMALIZATION • Three common situations where denormalization may be used Two entities with a one-to-one relationship A many-to-many relationship with nonkey attributes Reference data
- 5. A POSSIBLE DENORMALIZATION SITUATION: TWO ENTITIES WITH ONE-TO-ONE RELATIONSHIP
- 6. EXAMPLE
- 8. DESIGNING PHYSICAL FILES • Physical File: • A named portion of secondary memory allocated for the purpose of storing physical records • Tablespace – named set of disk storage elements in which physical files for database tables can be stored • Extent – contiguous section of disk space • Constructs to link two pieces of data: • Sequential storage • Pointers – field of data that can be used to locate related fields or records
- 9. FILE ORGANIZATIONS • Technique for physically arranging records of a file on secondary storage • Factors for selecting file organization: • Fast data retrieval and throughput • Efficient storage space utilization • Protection from failure and data loss • Minimizing need for reorganization • Accommodating growth • Security from unauthorized use
- 10. File Organization Continue… How the files are arranged on the disk. In modern RDBMS user do not have to design file organization but may be allowed to select a file- organization and its parameter. Access Method: How the data can be retrieved based on the file
- 11. TYPES OF FILE ORGANIZATIONS • Sequential • Indexed • Hashed
- 12. SEQUENTIAL • • The rows in the file are stored in sequence according to a primary key value • • Updating and adding records may require rewriting the file • • Deleting records results in wasted space
- 13. SEQUENTIAL FILE ORGANIZATION Records of the file are stored in sequence by the primary key field values
- 14. INDEXED FILE ORGANIZATION • • An indexed file contains records ordered by a record key . A record key uniquely identifies a record and determines the sequence in which it is accessed with respect to other records. . • The rows are stored either sequentially or non-sequentially and an index is created that allows software to locate individual rows Indexing approaches: • B-tree index • Bitmap index • Hash Index • Join Index
- 15. B- TREE INDEX Leaves of the tree are all at same level consistent access time
- 16. HASHED FILE ORGANIZATION • – The address for each row is determined using an algorithm
- 18. 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
- 19. CONTINUE… 5. When there are >100 values but not when there are <30 values 6. DBMS may have limit on number of indexes per table and number of bytes per indexed field(s) 7. Null values will not be referenced from an index 8. 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