The indices used by Firebird are B-trees , and they are bidirectional, but in practice this bidirectionality is not used, since the reverse direction is considered unreliable. This is due to the order of updates and how Firebird writes pages. As a result, reading in the opposite direction may skip index pages if this reading occurs simultaneously with the division of the index page.
See also Firebird for the database expert. Episode 3: Drive Consistency :
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