Function to emulate ON DUPLICATE KEY UPDATE functionality in Postgresql

merge_kvstore.psql

CREATE FUNCTION merge_kvstore(dkey character varying, ddata BYTEA) RETURNS VOID AS
$$
BEGIN
    LOOP
        -- first try to update the key
        UPDATE kvstore SET value = ddata WHERE key = dkey;
        IF found THEN
            RETURN;
        END IF;
        -- not there, so try to insert the key
        -- if someone else inserts the same key concurrently,
        -- we could get a unique-key failure
        BEGIN
            INSERT INTO kvstore(key,value) VALUES (dkey, ddata);
            RETURN;
        EXCEPTION WHEN unique_violation THEN
            -- Do nothing, and loop to try the UPDATE again.
        END;
    END LOOP;
END;
$$
LANGUAGE plpgsql;