postgresql - 表上的条件唯一约束？

Question

我有这张桌子：

CREATE TABLE mitg.tbl_gch_customers (
    pe character varying(10)NOT NULL,
    name character varying NOT NULL,
    city character varying,
    address character varying  NOT NULL,
    state character varying,
    zip character varying ,
    country character varying,
    phone character varying,
    "primary" character(1) NOT NULL
)

如何添加约束以确保将唯一行定义为唯一pe“标识符”AND "primary"= 'Y'？换句话说，一个客户行的每个数字"primary"只能有一个标记为“Y” 。pe但是这个约束必须允许多个"primary"= 'N' 客户具有相同的pe数量。