Indexed tables

For years, my friends used Informix with cobol. They used logical files, that are the same file (or table), but with different key fields. We don't know how they are, but access to logical files is more speedy when the search is on a fields that aren't key. Now I want to use ODBC from Windows client, and I use SQL (cobol doesn't use SQL). The problem is that I don't know how to use logial files, because I can use only fisical files. This is a problem, because speed from ODBC is the same of cobol when I use a SELECT with WHERE on keys fields, but is very slower when I use a SELECT with WHERE on not key fields. The problem is that cobol uses logical files and I don't. With SQL is there some of similar? How can Informix give me the same speed of cobol on a not key fields? Thank, Berna.