Partial key usage

PostGreSQL 7.4.3

I have a table that 'translates' codes between two types. The structure
is:
recno | integer | not null default
nextval('public.kah_kahxlate_recno_seq'::text)
kahcode | character(25) | not null
othercodetype | character varying(40) | not null
othercode | character varying(40) | not null
othercoden | numeric(20,0) |
Indexes:
"kah_kahxlate_cpk" primary key, btree (recno)
"ka_kahxlate_2" btree (othercodetype, othercode)
"kah_kahxlate_1" btree (kahcode, othercodetype)

What can happen is that the 'othercode' can be partial - so can be
accessed with LIKE - ie
SELECT kahCode FROM kah_kahXlate
WHERE otherCodeType = 'FRED'
AND otherCode LIKE 'ABC%';

This appears to use a sequential access according to the explain plan.
In Informix, to get around this the LIKE could be changed to address a
substring of the 'otherCode' - ie :
....
AND otherCode[1,3] = 'ABC'

This would then use the index.
I have tried making the PostGreSQL code:
AND SUBSTRING(otherCode FROM 1 FOR 3) = 'ABC'

But this still comes up with a sequential scan. Is there a way to force
an indexed read?

(I did prove that it was using a sequential scan by dropping the index
and retrying the query - same time - the explain plan is accurate)

Regards,

Steve Tucknott

ReTSol Ltd

DDI: 01903 828769