Hibernate 检索策略

检索策略

1 概述

在查询数据库时，通常可以从两个方面提升程序的性能：

①节约内存空间

当应用程序需要读取Customer对象的数据并从数据库中将Customer对象加载到内存中，如果同时加载所有关联的Order对象的集合，但是又不使用，那么这些关联的Order对象就白白浪费了许多内存空间。

②少发送SQL语句

每发送一次SQL语句就需要获取一个数据库连接，与数据库进行一次完整的交互过程，而访问数据库往往是一个非常耗时的操作，应尽量减少访问数据库的次数。

2 类级别的检索策略：

load()方法的行为

①默认情况下加载对象时使用延迟检索策略，仅返回代理对象

②使用立即检索时，在方法执行时就加载全部数据

< class name ="Customer" table ="CUSTOMERS" lazy ="false">

3 访问n的一端时的检索策略

①默认情况

加载Customer的时候，不会自动立即加载关联的Order集合

②延迟加载

在Customer的set元素内设置lazy=”false”后就会在加载Customer的同时发送加载Order集合的SQL语句。注意：这里虽然使用了立即加载策略但是并没有合并成一条SQL语句，而是使用了两条SQL语句

延迟检索的SQL

Hibernate:
select
customer0_.CUST_ID as CUST_ID1_0_0_,
customer0_.CUST_NAME as CUST_NAM2_0_0_
from
CUSTOMERS customer0_
where
customer0_.CUST_ID=?

在访问Order集合的数据之前

Hibernate:
select
orderset0_.CUST_ID_FK as CUST_ID_3_0_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_0_,
orderset0_.ORDER_NAME as ORDER_NA2_1_0_,
orderset0_.CUST_ID_FK as CUST_ID_3_1_0_
from
ORDERS orderset0_
where
orderset0_.CUST_ID_FK=?
ORDER01
ORDER02

在访问Order集合的数据之后

立即检索的SQL

Hibernate:
select
customer0_.CUST_ID as CUST_ID1_0_0_,
customer0_.CUST_NAME as CUST_NAM2_0_0_
from
CUSTOMERS customer0_
where
customer0_.CUST_ID=?
Hibernate:
select
orderset0_.CUST_ID_FK as CUST_ID_3_0_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_0_,
orderset0_.ORDER_NAME as ORDER_NA2_1_0_,
orderset0_.CUST_ID_FK as CUST_ID_3_1_0_
from
ORDERS orderset0_
where
orderset0_.CUST_ID_FK=?

在访问Order集合的数据之前

ORDER02

ORDER01

在访问Order集合的数据之后

③增强的延迟加载

使用lazy属性的extra值表示进一步推迟将数据加载到内存的时机，例如：查询关联的集合的长度时，只发送一条count函数SQL语句

Hibernate:
select
customer0_.CUST_ID as CUST_ID1_0_0_,
customer0_.CUST_NAME as CUST_NAM2_0_0_
from
CUSTOMERS customer0_
where
customer0_.CUST_ID=?
Hibernate:
select
count(ORDER_ID)
from
ORDERS
where
CUST_ID_FK =?
2

④batch-size

使用set元素的batch-size属性可以设置在内存中一次性初始化关联的Order集合的个数，减少发送SQL语句的次数。例如：内存中有5个Customer，batch-size=2

【注意：当lazy=时，batch-size设置无效】

Hibernate:
select
customer0_.CUST_ID as CUST_ID1_0_,
customer0_.CUST_NAME as CUST_NAM2_0_
from
CUSTOMERS customer0_
customer的个数：5
Hibernate:
select
orderset0_.CUST_ID_FK as CUST_ID_3_0_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_0_,
orderset0_.ORDER_NAME as ORDER_NA2_1_0_,
orderset0_.CUST_ID_FK as CUST_ID_3_1_0_
from
ORDERS orderset0_
where
orderset0_.CUST_ID_FK in (
?, ?
)

order集合长度：2

order集合长度：3

Hibernate:
select
orderset0_.CUST_ID_FK as CUST_ID_3_0_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_0_,
orderset0_.ORDER_NAME as ORDER_NA2_1_0_,
orderset0_.CUST_ID_FK as CUST_ID_3_1_0_
from
ORDERS orderset0_
where
orderset0_.CUST_ID_FK in (
?, ?
)
order集合长度：1
order集合长度：1
Hibernate:
select
orderset0_.CUST_ID_FK as CUST_ID_3_0_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_0_,
orderset0_.ORDER_NAME as ORDER_NA2_1_0_,
orderset0_.CUST_ID_FK as CUST_ID_3_1_0_
from
ORDERS orderset0_
where
orderset0_.CUST_ID_FK=?
order集合长度：0

⑤set元素的fetch属性

[1]select：默认值

[2]subselect：忽略batch-size的设置，以子查询的方式加载全部的Order集合。

Hibernate:
select
customer0_.CUST_ID as CUST_ID1_0_,
customer0_.CUST_NAME as CUST_NAM2_0_
from
CUSTOMERS customer0_
customer的个数：5
Hibernate:
select
orderset0_.CUST_ID_FK as CUST_ID_3_0_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_1_,
orderset0_.ORDER_ID as ORDER_ID1_1_0_,
orderset0_.ORDER_NAME as ORDER_NA2_1_0_,
orderset0_.CUST_ID_FK as CUST_ID_3_1_0_
from
ORDERS orderset0_
where
orderset0_.CUST_ID_FK in (
select
customer0_.CUST_ID
from
CUSTOMERS customer0_
)

order集合长度：2

order集合长度：3

order集合长度：1

order集合长度：1

order集合长度：0

[3]join时：会在加载Customer对象的时候，就“迫不及待”的查询关联的Order集合，这样的检索方式我们在Hibernate中称之为“迫切左外连接”。

注意：取值为join时，会忽略lazy属性的设置。但是fetch=join对HQL查询无效。

4 访问1的一端

①立即检索

在many-to-one元素中设置lazy=”false”

Tip：在many-to-one元素中lazy属性的可选值是：false、proxy、no-proxy，其中默认值是proxy。

Hibernate:
select
order0_.ORDER_ID as ORDER_ID1_1_0_,
order0_.ORDER_NAME as ORDER_NA2_1_0_,
order0_.CUST_ID_FK as CUST_ID_3_1_0_
from
ORDERS order0_
where
order0_.ORDER_ID=?
Hibernate:
select
customer0_.CUST_ID as CUST_ID1_0_0_,
customer0_.CUST_NAME as CUST_NAM2_0_0_
from
CUSTOMERS customer0_
where
customer0_.CUST_ID=?
ORDER01
CUST01

②迫切左外连接

将fetch属性设置为join即可

Hibernate:
select
order0_.ORDER_ID as ORDER_ID1_1_1_,
order0_.ORDER_NAME as ORDER_NA2_1_1_,
order0_.CUST_ID_FK as CUST_ID_3_1_1_,
customer1_.CUST_ID as CUST_ID1_0_0_,
customer1_.CUST_NAME as CUST_NAM2_0_0_
from
ORDERS order0_
left outer join
CUSTOMERS customer1_
on order0_.CUST_ID_FK=customer1_.CUST_ID
where
order0_.ORDER_ID=?
ORDER01
CUST01

③batch-size

当查询了一组Order，内存中存在多个Customer时，可以在Customer.hbm.xml文件的class元素中设置batch-size属性，决定加载一组Order集合时同时初始化Customer对象的个数。例如：内存中有4个Customer需要初始化，batch-size设置为3时：

Hibernate:
select
order0_.ORDER_ID as ORDER_ID1_1_,
order0_.ORDER_NAME as ORDER_NA2_1_,
order0_.CUST_ID_FK as CUST_ID_3_1_
from
ORDERS order0_
Hibernate:
select
customer0_.CUST_ID as CUST_ID1_0_0_,
customer0_.CUST_NAME as CUST_NAM2_0_0_
from
CUSTOMERS customer0_
where
customer0_.CUST_ID in (
?, ?, ?
)
CUST01
CUST01
CUST02
CUST02
CUST02
CUST03
Hibernate:
select
customer0_.CUST_ID as CUST_ID1_0_0_,
customer0_.CUST_NAME as CUST_NAM2_0_0_
from
CUSTOMERS customer0_
where
customer0_.CUST_ID=?
CUST04