Help on table indexing needed

Sirs, i have a informix (online) table with the following scheme. It contains the data of patient's stay in our hospital. ( izahl char(10) not null, # the identification of the patient station char(4) not null, # the ward a_datum date not null, # the start-date in the hospital e_datum date, # the leaving date ..... some other stuff deleted ....... ); My main query is to ask: which person were at a day or at a time-interval on station xx In Informix 4GL the code is: let ss = "select * from stationaer", " where ", " (stationaer.e_datum is null or stationaer.e_datum >= ?)", " and stationaer.a_datum <= ? ", " and stationaer.station = ?" This code is rather slow and I don't not know how to set the indices. I indexed on every of this 4 field, but I seem the query resolves in a sequential search. INFORMIX-VERSION is 4.1. Can You help me please. -- Dr. med. dipl.-math Dieter Becker Tel.: (0 / +49) 6841 - 16 3046 Medizinische Universitaets- und Poliklinik Fax.: (0 / +49) 6841 - 16 3369 Innere Medizin III D - 66421 Homburg / Saar Email: becker@med-in.uni-sb.de