Database design question

Thanks to those who responded to my specific query last time. I'm going to ask the same question in the abstract: say I have a table with a million rows. One column is an indicator, which will almost always be null. However, it is *very* important to be able to quicly identify the 5 rows (out of the million) that have the indicator set. Some DBMSs, Adabas for example, would allow the column to be set up as a "null-suppressed index", that is, only non-null values would have an index entry built for them. This is extremely efficient. How would you do it in a relational design? Thanks Neil Truby Chase Bank Bournemouth, UK