Some time ago I made test with "in memory index".
This was implemented as set of UDF and triggers and Stored Procedures.

Select on FB 1.5 was 2x faster with UDF function, that converts
a value to DB_KEY.
For index I use linear array of records (value, DB_KEY) and when I need
sort
I use quick sort.

It is can be used for static tables (less changes) but very offen reads.
This index should be exists only into memory and is fully bi-directional
(no limitation for backward order).

Slavek