这个问题通常由于子进程未能正确地关闭数据库连接而引起。应该在进程完成后显式地关闭连接，并在主进程的所有子进程结束后调用Process.join()。下面是一个示例：

from multiprocessing import Process
import psycopg2

def worker():
    conn = psycopg2.connect(database="my_db", user="my_user", password="my_password", host="localhost")
    # do something with the connection
    conn.close()

if __name__ == '__main__':
    # create/process the workers here
    processes = []
    for i in range(4):
        p = Process(target=worker)
        processes.append(p)
        p.start()

    # wait for all workers to complete
    for p in processes:
        p.join()

    # close the database connection in the main process
    conn = psycopg2.connect(database="my_db", user="my_user", password="my_password", host="localhost")
    conn.close()