For example, the
Gender column in the PERS table will either be NULL, M, or F. The bitmap index on the Gender
column will have only three bitmaps stored in the index. On the other hand, a bitmap index on
the Last_Name column will have close to the same number of bitmap strings as rows in the table
itself! The queries looking for a particular last name will most likely take less time if a full table
scan is performed instead of using an index. In this case, a traditional B-treenon-unique index
makes more sense.
A variation of bitmap indexes called bitmap join indexes creates a bitmap index on a table
column that is frequently joined with one or more other tables on the same column. This provides
tremendous benefits in a data warehouse environment where a bitmap join index is created on a
fact table and one or more dimension tables, essentially pre-joining those tables and saving CPU
and I/O resources when an actual join is performed.
NOTE
Bitmap indexes are only available in the Enterprise Edition of
Oracle 11g.
Views
Views allow users to see a customized presentation of the data in a single table or even a join
between many tables. A view is also known as a stored query??”the query details underlying the
view are hidden from the user of the view. A regular view does not store any data, only the definition,
and the underlying query is run every time the view is accessed. Extensions to a regular view,
called a materialized view, allows the results of the query to be stored along with the definition
of the query to speed processing, among other benefits.
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