I have this table with a cloumn A whose value may be null. I'd like to create an index on A, so that I can efficiently access rows using WHERE A=? statements. I would never of course need to access rows whose A value is null.

For better performance, can I tell Derby not to index rows whose A value is null? If so, how? Should I create at all an index on a column which is not defined as NOT NULL? Will it improve performance?

- Iris