1. Database Management System
Lecture - 39
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Index Classification
Clustered vs. unclustered:
If order of data records is the same as order of index data entries, then called clustered index.
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Index Classification
A file can be clustered on at most one search key.
Cost of retrieving data records through index varies greatly based on whether index is clustered or not!
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4. Clustered vs. Unclustered Index
Index entries
direct search for
data entries
Data
entries
Data
entries
(Index File)
(Data file)
Data Records
Data Records
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When to Use Indexes
Useful on large tables
Required on PK
Useful for attributes appearing in where, order by and group by clauses
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When to use
Significant variety in values
Check the limit of indexes with your DBMS
Make your decision very much justifiable; indexes introduce overhead
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Indexes Summary
Useful tool to speedup the data access
Can be unique or non-unique
Can be implemented through different techniques
Be careful; they involve overhead
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Views
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Definition
A view is defined to combine certain data from one or more tables for different reasons
A view is like a window through which we can see data from one or more tables
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Why Views?
Security
Show the data to a users’ group that is necessary/required for them
Give only necessary authorizations
No concern with rest of the data
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Why Views?
Efficiency
Part of a query that is frequently used (a subquery), define it as a view
The view definition will be stored and will be executed anytime anywhere view is referred
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Why Views?
Join columns from multiple tables so that they look like a single table
Aggregate information instead of supplying details
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13. Characteristics of Views
Not exactly the external views of 3-level schema architecture
Major Types
Dynamic
Materialized
Partitioned
Simple/complex
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Dynamic Views
Data is not stored for the views
Definition is stored in the schema
Executed every time view is referred
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15. Defining Dynamic Views
CREATE VIEW [ < database_name > . ] [ < owner > . ] view_name [ ( column [ ,...n ] ) ] [ WITH ENCRYPTION | SCHEMABINDING [ ,...n ] ] AS select_statement [ WITH CHECK OPTION ]
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16. Database Management System
Lecture - 39
© Virtual University of Pakistan