What is Index?
An index is a performance-tuning method of allowing faster retrieval of records. An index creates an entry for each value that appears in the indexed columns. By default, Oracle creates B-tree indexes.
Oracle’s two major index types are Bitmap indexes and B-Tree indexes. B-Tree indexes are the regular type that OLTP systems make much use of, and bitmap indexes are a highly compressed index type that tends to be used primarily for data warehouses.
What is Bitmap Index?
A bitmap index is an indexing method that can provide both performance benefits and storage savings. Bitmap indexes are particularly useful for data warehousing environments because data is usually updated less frequently and ad hoc queries are more common.
Bit-mapped indexes are best for “low-cardinality” data (such as a column to indicate a person’s gender, which contains only two possible values: MALE and FEMALE).
Characteristic of Bitmap Indexes:
1. For columns with very few unique values (low cardinality)
2. Tables that have no or little insert/update are good candidates (static data in warehouse)
3. Stream of bits: each bit relates to a column value in a single row of table.
What is B-Tree Index?
B-tree indexes are usually created on columns containing the most unique values. It is an ordered set of entries in which each entry a search key value and a pointer to a specific row with that value. When a server finds a search key value matching a constraint, the pointer is used to fetch the row.