Firebird indexes are uni-directional, either ascending or descending.
To efficiently retrieve max or high values, I need a DESCENDING INDEX;
for min or low values, an ASCENDING INDEX.

Could you briefly sketch out for the interested layman why Firebird's
indexes are directional (history?), what are the advantages, and what
particularity of the index structure makes it suitable for just one
direction, and not both?

Second point of interest, what kind of indexes and with what
directionality are used to back up the following CONSTRAINTs:

* PRIMARY KEY
* FOREIGN KEY
* UNIQUE

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Michael Ludwig