How many indexes are there in Oracle? What is the purpose of each?

1. B-tree Index
The most common index type in an Oracle database is the B-tree index, the B-tree index, named after its scientific structure with the same name. CREATE
When the index statement is created, the default is to create the B-tree index. No special provisions can be used in any situation.
2. Bitmap indexing (bitmap index)
A bitmap index is specific to a case where there are only a few enumeration values for the column, such as the gender field, which indicates fields such as 0 and 1.
3. Function-based indexing
For example, when a field is frequently queried for functions, it is worthwhile to build a function index at this point.
4. Partitioned indexes and global indexes
These 2 are used for partitioned tables. The former is an intra-partition index, and the latter is a full table index
5. Reverse index (REVERSE)
This index is not common, but is particularly useful for specific situations, such as a varchar (5) Bit field (employee number) with a value
(10001,10002,10033,10005,10016.)
In this case, the default index is too dense to take advantage of good server parallelism
But after the reverse 10001,20001,33001,50001,61001 has a very good distribution, can effectively use the good parallel operation.
6.HASH Index
A hash index can be the quickest way to access data in a database, but it also has its drawbacks. The number of different values on the cluster key must be known before the hash cluster is created. You need to specify this value when creating a hash cluster. Hash clusters must be used to use hash indexes.

How many indexes are there in Oracle? What is the purpose of each?