> > >>> Does indexing a very big table (about 5Mio records) on the columnA
> > >>> which can hold only values Y,N,<NULL> make sense?
> > >> Yes, that's what bitmap indexes were made for.

FYI , you can also index only the N ( or Y) value's in the column.
This is done if e.g. the boolean is an indicator whether a row has
gone through a processing run or not, and the application just need to
find the N values.
Bitmap indexes are not ok if the table is updated intensively.

Helma