为什么在hibernate配置文件中为连接池设置10？size = 1是内置的size。为什么需要增加大小？

<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>
     ....

       <property name="connection.pool_size">10</property>

     ....
    </session-factory>

</hibernate-configuration>