indexing for query speed - index row size exceeding btree maximum

Hi, I have a table with 8M rows. One of the fields is of type text and I
wanted to create an index on it to improve query times. Now this field
is a single string (ie not a piece of normal text) and is really an
identifier (< 100 chars).  I envisage queries like

select cid from tableName where fieldName = 'XYZ ... ';

So I had done something like

create index someName on tableName (fieldName);

However this returned with an error:

ERROR: index row size 2848 exceeds btree maximum, 2713

I noted that some other posters have faced this problem, but most of the
replies requested info regarding what the index was to be used for. I
also tried following one example where the index was created on the MD5
hash of the field being indexed - but this did not make my query times
significantly faster.

Is there a way for me to generate an index on this field so that my
query times can be reduced?

Thanks,

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Rajarshi Guha <rguha@indiana.edu>
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A method of solution is perfect if we can forsee from the start,
and even prove, that following that method we shall attain our aim.
-- Leibnitz