We introduce many of them here, but see Chapter 10 for
more detailed explanations of products and features specific to data warehousing
and business intelligence.
Database Performance Features | 21
Bitmap indexes
Oracle added support for stored bitmap indexes to Oracle 7.3 to provide a fast way
of selecting and retrieving certain types of data. Bitmap indexes typically work best
for columns that have few different values relative to the overall number of rows in a
table.
Rather than storing the actual value, a bitmap index uses an individual bit for each
potential value with the bit either ???on??? (set to 1) to indicate that the row contains the
value or ???off??? (set to 0) to indicate that the row does not contain the value. Bitmap
indexes are described in more detail in Chapter 4.
Star query optimization
Typical data warehousing queries occur against a large fact table with foreign keys to
much smaller dimension tables. Oracle added an optimization for this type of star
query in Oracle 7.3. Performance gains are realized through the use of Cartesian
product joins of dimension tables with a single join back to the large fact table.
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