Postgres 函数语法 near at IF

问：

我正在编写一个Postgres函数，如下所示

CREATE OR REPLACE FUNCTION dbo.get_managing_owner(p_imo integer, p_charter_id integer)
RETURNS TABLE(managing_owner character varying) 
LANGUAGE plpgsql
AS $function$
BEGIN
 RETURN QUERY
IF p_charter_id = 0
THEN
    SELECT managing_owner
    from dbo.vessel
    where imo=p_imo
    union
    SELECT managing_owner
    from dbo.charter c
    where imo=p_imo;
ELSE 
    SELECT managing_owner
    from dbo.vessel v
    inner join dbo.charter c2
    on v.imo =c2.imo
    where c2.id =p_charter_id
    union
    SELECT managing_owner
    from dbo.charter c
    where id=p_charter_id;
END IF;
end;
$function$
;

我在 IF 附近遇到语法错误，我在这里找不到任何问题。有人可以帮我解决这个问题吗？