> I am not sure what you mean by 'shape' of the data.
> Can you clarify a bit??

If you have an integer field that contains no duplicates, and then you
update 95% of the records to the value 3, the usefulness of that index
plummets, however Firebird will consider it as a really useful index
until the statistics are recalculated (a manually fired off task).

That is an extreme case, but a query plan is generated by using these
statistics to estimate the cost of each path. If lots of indices have
statistics that do not accurately reflect the usefulness, there is a
pretty good chance a bad plan will be selected.

Adam