There are some things you should always index:
- Primary Keys - these are given an index automatically (unless you specify a suitable existing index for Oracle to use)
- Unique Keys - these are given an index automatically (ditto)
- Foreign Keys - these are not automatically indexed, but you should add one to avoid performance issues when the constraints are checked
After that, look for other columns that are frequently used to filter queries: a typical example is people's surnames.
