ERROR: DefineIndex: index function must be marked iscachable

Hi!

Who can help me on that?

First of all, my envoronment is:
Linux netlab142.prodam 2.4.8-26mdk #1 Sun Sep 23 17:06:39 CEST 2001
i686 unknown
pg_ctl (PostgreSQL) 7.2.

Problem: ERROR: DefineIndex: index function must be marked iscachable
by executing:

create index bt_proposta_f01 on proposta
using btree (func_cod_secretaria(nr_proponente));

Where nr_proponte is a integer type column.

The function is:

CREATE OR REPLACE FUNCTION func_cod_secretaria(INTEGER) RETURNS INTEGER AS '
DECLARE
v_nr_proponente ALIAS FOR $1;
BEGIN
return TRUNC(v_nr_proponente/100000,0)*100000
END;
' LANGUAGE 'plpgsql';

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