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For instance, once you??™ve completed an optimal logical database
design, youmu st go back and consider what indexes youshou ld add to improve the
anticipated performance of the database and whether youshou ld designate any
tables as part of a cluster or hash cluster.
Since adding these types of performance-enhancing data structures doesn??™t affect the
logical representation of the database, youcan always make these types of modifications
later when yousee the way an application uses the database in test mode or in
production.
Constraints
A constraint enforces certain aspects of data integrity within a database. When you
add a constraint to a particular column, Oracle automatically ensures that data violating
that constraint is never accepted. If a user attempts to write data that violates a
constraint, Oracle returns an error for the offending SQL statement.
Constraints may be associated with columns when you create or add the table containing
the column (via a number of keywords) or after the table has been created
with the SQL command ALTER TABLE.


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