sql-插入每个分区的无间隙序列号

我有一个带有触发器的表，它为每个客户设置一个订单号，我经常在该表中遇到一个唯一键：

SQLSTATE[23505]: Unique violation: 7
ERROR: duplicate key value violates unique constraint "order_customer_unique"

触发器定义为BEFORE INSERT ON orders FOR EACH ROW.

有什么技术可以解决表中的并发问题吗？
理想情况下，每个客户的数字是一个序列，不会丢失数字。

CREATE OR REPLACE FUNCTION "public"."trigger_set_order_customer_number"()
  RETURNS "pg_catalog"."trigger" AS
$BODY$
BEGIN
   NEW.sequence = (
      SELECT COALESCE( MAX ( order.sequence), 0 ) + 1
      FROM orders
      WHERE orders.customer_type = NEW.customer_type
      AND orders.customer_id = NEW.customer_id
      AND orders.model_id = NEW.model_id LIMIT 1);
   RETURN NEW;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100