Beginning Oracle Database 11g Administration From Novice to Professional

CHAPTER 7 PHYSICAL DATABASE DESIGN 133
Unique vs. Non- Unique Indexes
Sometimes, the collection of indexed values should not include duplicates. For example,
no two employees should have the same employee ID number. You can use the
clause of the command to enforce the uniqueness requirement, as in the
following example:
Concatenated Indexes
Consider the following query, which retrieves all red cars registered in the state of
California: . The query
would probably benefit from an index of states or an index of colors, but an even more
selective index would be an index of state and color combinations. The following command
creates an index of state and color combinations and names it :
Oracle is capable of using the information in a concatenated index even if all relevant
data items are not restricted in the SQL query. Instead of separate indexes, one of
states and one of colors, let us suppose that a concatenated index has been created as in
the previous paragraph. Also, suppose that the query specifies only the state or the color
but not both. Oracle is capable of using the concatenated index even in such a case. For
example, consider the query . In this case, it
makes perfect sense to use the concatenated index described in the previous paragraph.
Now consider the query . Because color
is not the leading item in the concatenated index, it might appear at first sight that the
concatenated index is not useful. However, Oracle can check the index 50 times, once for
each state in the union, and thus identify rows satisfying both restrictions. It makes sense
to do so because indexes are relatively compact objects, compared to tables. Also, database
indexes are structured for easy lookup, just as the index of keywords at the back of
this book is sorted in alphabetical order.
Function-Based Indexes
Indexes of the type we have considered so far are not as useful if the restrictions listed in
the query include anything more complex than simple equality and inequality checks.
For example, an index of salaries would not help if we are trying to identify employees for
whom the total of salary and bonus is greater than a certain amount. Therefore, Oracle
provides the ability to create indexes of the results of expressions. For example, the following
command creates an index of the total of salary and bonus: