The following documents are worth reading:
IBM Informix Dynamic Server Administrator's Guide: http://publibfp.boulder.ibm.com/epubs/pdf/ct1ucna.pdf
IBM Informix Trusted Facility Guide: http://publibfp.boulder.ibm.com/epubs/pdf/ct1tbna.pdf
IBM Informix Guide to SQL: http://publibfi.boulder.ibm.com/epubs/pdf/ct1sqna.pdf