Hi Prachi,
Fields that are automatically indexed in Salesforce are:
- RecordTypeId
- Division
- CreatedDate
- Systemmodstamp (LastModifiedDate)
- Name
- Email (for contacts and leads)
- Foreign key relationships (lookups and master-detail)
- The unique Salesforce record ID, which is the primary key for each object.
Salesforce also supports custom indexes on custom fields, with the exception of multi-select picklists, text area (long), text area (rich), non-deterministic formula fields, and encrypted text field