Primary vs. secondary: If search key contains primary key, then called primary index.
- Unique index: Search key contains a candidate key.
Clustered vs. unclustered: If order of data records is the same as, or `close to’, order of data entries, then called clustered index.
- A file can be clustered on at most one search key.
- Cost of retrieving data records through index varies greatly based on whether index is clustered or not!