Firebird and InterBase have single direction indexes for your data safety.

In most database index nodes are doubly linked to allow bi-directional scans. http://is.gd/8CMb7w, however not for InterBase and FireBird, there the reverse link isn’t used because it can be inconsistent due to write order of index pages.

The result is that in Firebird and InterBase, indexes are single-directional (either ascending or descending).

This is for your safety: it guarantees index consistency, even if because of EMP, your machine suddenly reboots after your tank fired a missile.

–jeroen

via Twitter / Avalanche1979: @SQLPerfTips For Firebird the ….

(Wow, did I really wrote 1200 blog posts?)