FULL JOIN is only supported with merge-joinable join conditions

I try to port application to PostgreSQL 8.1+

The following query runs OK in VFP but causes error in Postgres

FULL JOIN is only supported with merge-joinable join conditions

How to fix ?

Andrus.

SELECT
ametikoh.Nimetus as ametikoht,
Isik.nimi,
Isik.eesnimi,
koosseis.kogus,
COALESCE( iandmed.Kuluobj, koosseis.objekt1) as osakond

FROM iandmed
FULL JOIN koosseis ON iandmed.ametikoht=koosseis.ametikoht
AND iandmed.Kuluobj= koosseis.objekt1
AND iandmed.AmetiKoht is not null
JOIN ametikoh ON COALESCE(iandmed.ametikoht,koosseis.ametikoht)=
ametikoh.AmetiKoht
LEFT JOIN isik ON iandmed.isik=isik.isik
WHERE true
ORDER BY 1,2

Revelant pars of table structures are:

CREATE TABLE iandmed
(
reanr integer NOT NULL DEFAULT nextval('iandmed_reanr_seq'::regclass),
isik character(10) NOT NULL,
miskuup date,
plopp date,
summavrt numeric(12,2),
kuluobj character(10),
ametikoht numeric(7),
CONSTRAINT iandmed_pkey PRIMARY KEY (reanr)
) ;

CREATE TABLE koosseis
(
id numeric(7) NOT NULL,
ametikoht numeric(7) NOT NULL,
objekt1 character(10) NOT NULL,
kogus numeric(4) NOT NULL DEFAULT 0,
algus date,
lopp date,
CONSTRAINT koosseis_pkey PRIMARY KEY (id)
);