Indexing NON-NULL values only

Is it possible with SE or Online (and if so which versions) to create an index on a column such that only non-null values are indexed? For example, if a column has a high proportion of null values, it would be nice to create an index on the column that results in fast searches when a non-null value is specified in the where clause. I ask because in the back of my mind I've got a nasty feeling that a normal index can't handle more than 65535(6?) duplicate entries for the same key value. For a column which contains mostly nulls, this limit could be easily reached. This could well be a restriction that applied only to old versions of Informix products - I don't know. Any assistance would be appreciated ... -- | Steve Chell steve@fujitsu.co.nz | | Fujitsu NZ Ltd, Auckland, New Zealand FAX: +64.9.3562437 |