1. Bitmap indexes are most appropriate for columns having low distinct values—such as GENDER, MARITAL_STATUS, and RELATION. This assumption is not completely accurate, however. In reality, a bitmap index is always advisable for systems in which data is not frequently updated by many concurrent systems. In fact, as I'll demonstrate here, a bitmap index on a column with 100-percent unique values (a column candidate for primary key) is as efficient as a B-tree index.
2. When to Create an Index
3. You should create an index if:
4. A column contains a wide range of values
5. A column contains a large number of null values
6. One or more columns are frequently used together in a WHERE clause or a join condition
7. The table is large and most queries are expected to retrieve less than 2 to 4 percent of the rows
By default if u create index that is nothing but b-tree index.
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