> This means, that performance-problems will only happen on insert,
> update and delete and not for a select ?

That depends on exactly how IB stores the index data in its data pages
and I dont know the details - but I would expect *simple* selects to
be mostly uneffected. In more complex selects (joins etc) there may
be other issues.

> What means high or low selectivity of a index ?

How selective is the index when given a full input value (all fields
of the index specified).

A highly selective index will return very few records. For example
the primary key always has maximum selectivity - it returns on only
record for any given input. (IOW It is very selective.)

A low selectivity index may return very many records. (IOW Its not
very selective.)


At what point low selectivity becomes a problem depends on many
things, such as; how many records in the table, how large the
database pages, how large the index values etc.


HTH

Geoff Worboys
Telesis Computing