Using Postgres db, and created a table with composite keys.

CREATE TABLE composite_key
(
ou_id character varying(3) NOT NULL,
product_name character varying(15) NOT NULL,
product_price numeric(10,2),

CONSTRAINT composite_key_pkey PRIMARY KEY (ou_id, product_name)
)
WITH (
OIDS=FALSE
);
ALTER TABLE composite_key OWNER TO postgres;
INSERT INTO composite_key VALUES ('1','1',1);
INSERT INTO composite_key VALUES ('1','2',1.20);
INSERT INTO composite_key VALUES ('2','1',2.10);

Deploy the VDB and use SOAP UI to do PUT (aka SQL UPDATE) generates attached odataerror.txt