How can i create a function based index on a varchar2 field that may contain a not valid number

Hello,

we must tune a third party application. So we cant change the table or views they use. But we are allowed to create new indexes.

To understand where the problem comes from we have two applications, the first transfers data to the second, and the

source table has a number key, the second application and destination table has a varchar2 key. The application that have to be tuned uses this view:

  CREATE OR REPLACE FORCE EDITIONABLE VIEW "PROD_FIN_100"."SOPPROKONTO" ("BUCHUNGSNUMMER", "KLASSE", "KONTONUMMER", "BELEGART", "INTERNEBELEGNUMMER", "BELEGDATUM", "SOLL", "HABEN", "BETRAG") AS

  SELECT bu.dvbuchungsnummer,

          TRIM (CAST (k.klasse AS VARCHAR2 (1))),