B+Tree Hash Table BitMap T-Tree
By default TimesTen uses B+Tree. The original version (SmallBase) from the 1990s supported T-Trees. TimesTen still supports T-Tree indexes but the administrator has to request for the DBMS to use them.
http://www.oracle.com/us/products/database/timesten/index.html
https://docs.oracle.com/database/timesten-18.1/
HP Labs
1996
SmallBase
Oracle