Best practice has always been not to create indexes on highly duplicate data. Scanning the entire table for a common value may be quicker, and updating an index can be very costly when many pointers to rows with the same value are spread over several pages. A work-around when an index is essential in this scenario is to extend the index with a more selective column, but this obviously makes it bigger.
For the first time, a far better solution was provided in IDS 14.10.FC2. It’s known as Informix 14.10 Partial Indexes
Partial Indexes (click for the relevant page in the IBM Knowledge Center).
In this article, we will demonstrate how to identify where such indexes might be appropriate, how to create them, and how much smaller they can potentially be.