In a data warehouse where fact tables become very large, bitmap
indexes created on foreign key indexed dimensional values can equate to
thousands of values. However, the ratio of fact to dimensional table rows
still makes a bitmap index a viable option. Thus, low cardinality is a relative
term. In other words, creating a bitmap index on a male or female valued
column in a table containing two rows is completely pointless!
Figure 3.4
A compressed
composite column
BTree index.
Figure 3.5
A bitmap index.
60
3.1
The Basics of Indexing
Bitmap Performance
In general, a bitmap index occupies less physical space ...
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