Sequential scan vs Index

Informix seems to forget his own indices and use sequential searches. Here is a sqexplain.out for a special search: QUERY: ------ select unique patient.* from stationaer, patient where stationaer.izahl = patient.izahl and stationaer.station = ? and stationaer.e_datum >= ? and stationaer.a_datum <= ? order by 2, 4 Estimated Cost: 91 Estimated # of Rows Returned: 1 Temporary Files Required For: Order By 1) db.patient: SEQUENTIAL SCAN 2) db.stationaer: INDEX PATH Filters: (db.stationaer.station = 'M306' AND (db.stationaer.e_datum >= '03/04/1994' AND db.stationaer.a_datum <= '03/04/1994' ) ) (1) Index Keys: izahl Lower Index Filter: db.stationaer.izahl = db.patient.izahl The tables were as follows: stationaer: Column name Type izahl char(10) aufnr integer station char(4) a_grund char(1) a_datum date a_uhrzeit smallint a_ueberw integer e_grund char(1) e_datum date e_uhrzeit smallint e_ueberw integer Index name Owner Type Cluster Columns stat_c1 db dupls No station e_datum ix296_1 db dupls No izahl and patient: Column name Type izahl char(10) ......... Index name Owner Type Cluster Columns pn_iz db unique No izahl ..... I cannot understand why Informix uses sequential scan for the table patient. Any Ideas?? Dieter -- 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