表上的条件唯一约束？

如何解决表上的条件唯一约束？

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，"primary" 客户行中的每个 pe 编号只能有一个标记为“Y”。但此约束必须允许多个 "primary" = 'N' 个客户具有相同的 pe 编号。