Experiences with database extensibility

The existing server extensibilities in modern DBMSs have been critical in our company's development of software products that improve database performance for certain scientific computing applications. We are planning to develop other products that will utilize an extensible database engine, and so we would be grateful for feedback on the following questions as they pertain to your experience in developing Oracle applications:

1. What types of extensibility (i.e., data types, functions/ methods, indexing, etc.) are available and convenient to use in practice?
2. What types of extensibility (possibly already available in other DBMSs) are currently missing in Oracle?
3. To what extent was your choice of Oracle as a development platform based primarily on its extensibility features?