where rdb$relation_id < 10
order by rdb$relation_id
PLAN (RDB$RELATIONS ORDER RDB$INDEX_1 INDEX (RDB$INDEX_1))
But it should be:
PLAN (RDB$RELATIONS ORDER RDB$INDEX_1)
Because the WHERE condition should just set the upper threshold key and thus limit the navigational scan. A prior regular index scan is completely redundant in this case.
Generally speaking, the same index should never appear in both ORDER and INDEX parts of the same plan item. This particular issue is a regression introduced in v2.0, however other aspects of this bug are visible in all FB versions.