我们面临一对一的关联问题，NHProf显示选择N + 1 警报，更改标准以使用FetchMode.Join不解析N + 1 问题。以下是详细信息。
软件版本详细信息： NH版本：1.2 .net版本：3.5 DB：Oracle 11g 二级缓存：已启用
Hbm文件和类实体。
描述：'Sample'实体可以包含0或1'关联' 实体。 “关联”实体具有来自的外键约束 '样本'实体。
Sample.hbm.xml
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="NHSample" namespace="NHSample">
<class name="Sample" table="SAMPLE" proxy="Sample"
polymorphism="explicit" >
<id name="Id" type="Decimal" unsaved-value="-1">
<column name="SA_ID" sql-type="NUMBER" not-null="true"
unique="true"/>
<generator class="sequence">
<param name="sequence">SA_ID_SEQ</param>
</generator>
</id>
<property name="SampleName" type="String">
<column name="SAMPLE_NAME" length="100" sql-type="VARCHAR2" not-
null="false"/>
</property>
<one-to-one name="Association" class="Association" property-
ref="SampleAssociated" cascade="all-delete-orphan"/>
</class>
</hibernate-mapping>
Sample.cs
namespace NHSample
{
public class Sample
{
public virtual decimal Id
{
get ;
set ;
}
public virtual string SampleName
{
get ;
set ;
}
public virtual Association Association
{
get ;
set ;
}
}
}
Association.hbm.xml
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="NHSample" namespace="NHSample">
<class name="Association" table="ASSOCIATION">
<id name="Id" type="Decimal" unsaved-value="-1">
<column name="AS_ID" sql-type="NUMBER" not-null="true"
unique="true"/>
<generator class="sequence">
<param name="sequence">AS_ID_SEQ</param>
</generator>
</id>
<property name="AssociationName" type="String">
<column name="ASSOCIATION_NAME" length="100" sql-type="VARCHAR2" not-
null="false"/>
</property>
<many-to-one name="SampleAssociated" class="Sample">
<column name="SA_ID" sql-type="NUMBER" not-null="true"/>
</many-to-one>
</class>
</hibernate-mapping>
Association.cs
namespace NHSample
{
public class Association
{
public virtual decimal Id
{
get ;
set ;
}
public virtual string AssociationName
{
get ;
set ;
}
public virtual Sample SampleAssociated
{
get ;
set ;
}
}
}
没有FetchMode的条件=用于访问示例实体的连接：
class Program
{
static void Main(string[] args)
{
using (ISession session = SessionFactory.OpenSession())
{
var electedIds = new List<decimal>() { 1, 2, 3 };
ICriteria criteria =
session.CreateCriteria(typeof(Sample));
criteria.Add(Expression.In("Id", electedIds));
var list = criteria.List();
}
Console.ReadKey();
}
}
当执行Criteria.List（）时，NHProf显示N + 1警报。 以下是NHProf显示的SQL语句。
- 声明＃1
SELECT this_.SA_ID as SA1_0_1_,
this_.SAMPLE_NAME as SAMPLE2_0_1_,
associatio2_.AS_ID as AS1_1_0_,
associatio2_.ASSOCIATION_NAME as ASSOCIAT2_1_0_,
associatio2_.SA_ID as SA3_1_0_
FROM SAMPLE this_
left outer join ASSOCIATION associatio2_
on this_.SA_ID = associatio2_.SA_ID
WHERE this_.SA_ID in (1 /* :p0 */,2 /* :p1 */,3 /* :p2 */)
- 陈述＃2
SELECT associatio0_.AS_ID as AS1_1_0_,
associatio0_.ASSOCIATION_NAME as ASSOCIAT2_1_0_,
associatio0_.SA_ID as SA3_1_0_
FROM ASSOCIATION associatio0_
WHERE associatio0_.SA_ID = 1 /* :p0 */
- 陈述＃3
SELECT associatio0_.AS_ID as AS1_1_0_,
associatio0_.ASSOCIATION_NAME as ASSOCIAT2_1_0_,
associatio0_.SA_ID as SA3_1_0_
FROM ASSOCIATION associatio0_
WHERE associatio0_.SA_ID = 2 /* :p0 */
- 陈述＃4
SELECT associatio0_.AS_ID as AS1_1_0_,
associatio0_.ASSOCIATION_NAME as ASSOCIAT2_1_0_,
associatio0_.SA_ID as SA3_1_0_
FROM ASSOCIATION associatio0_
WHERE associatio0_.SA_ID = 3 /* :p0 */
使用NHProf网站建议修改cirteria使用 FETCHMODE.Join：
class Program
{
static void Main(string[] args)
{
using (ISession session = SessionFactory.OpenSession())
{
var electedIds = new List<decimal>() { 1, 2, 3 };
ICriteria criteria =
session.CreateCriteria(typeof(Sample));
criteria.SetFetchMode("Association", FetchMode.Join);
criteria.Add(Expression.In("Id", electedIds));
var listByIds = criteria.List();
}
Console.ReadKey();
}
}
NHProf仍显示N + 1警报，以下是SQL查询。
- 声明＃1
SELECT this_.SA_ID as SA1_0_1_,
this_.SAMPLE_NAME as SAMPLE2_0_1_,
associatio2_.AS_ID as AS1_1_0_,
associatio2_.ASSOCIATION_NAME as ASSOCIAT2_1_0_,
associatio2_.SA_ID as SA3_1_0_
FROM SAMPLE this_
left outer join ASSOCIATION associatio2_
on this_.SA_ID = associatio2_.SA_ID
WHERE this_.SA_ID in (1 /* :p0 */,2 /* :p1 */,3 /* :p2 */)
- 陈述＃2
SELECT associatio0_.AS_ID as AS1_1_0_,
associatio0_.ASSOCIATION_NAME as ASSOCIAT2_1_0_,
associatio0_.SA_ID as SA3_1_0_
FROM ASSOCIATION associatio0_
WHERE associatio0_.SA_ID = 1 /* :p0 */
- 陈述＃3
SELECT associatio0_.AS_ID as AS1_1_0_,
associatio0_.ASSOCIATION_NAME as ASSOCIAT2_1_0_,
associatio0_.SA_ID as SA3_1_0_
FROM ASSOCIATION associatio0_
WHERE associatio0_.SA_ID = 2 /* :p0 */
- 陈述＃4
SELECT associatio0_.AS_ID as AS1_1_0_,
associatio0_.ASSOCIATION_NAME as ASSOCIAT2_1_0_,
associatio0_.SA_ID as SA3_1_0_
FROM ASSOCIATION associatio0_
WHERE associatio0_.SA_ID = 3 /* :p0 */
有关如何克服此附加SQL语句和N + 1的任何想法 在这种情况下的问题。
谢谢。
答案 0 :(得分：3)
这是NHibernate中的一个已知问题;加入提取并不能防止在各种情况下出现N + 1问题。请注意，使用连接提取的第一个查询实际上包括连接和映射字段; NHibernate正在根据映射构建正确的查询，但是没有将该信息传递到已连接的子集合中，因此它只返回并运行N个查询来拉动子项。
您可以在此处查看JIRA项目：https://nhibernate.jira.com/browse/NH-2534。 NHibernate团队已经开始进攻，并且积压了积压的错误，这个标记很重要（我会称之为;我正在等待一个新版本来解决我自己的N + 1问题，我的评论就是证明了这一点对于这个确切的错误），所以我猜他们一旦得到它就会得到修复。
就个人而言，我不介意使用2遍查询解决方案，默认情况下可以使用外键查询在映射中将NHibernate告知延迟加载。目前，您只能通过构建自己的IQuery来自行加载子项来指定此行为。由NH生成的惰性代理使用外键查询检索到的ID进行了预初始化，但随后它通过ID一次性提取一个记录的完整数据。那太疯狂了;如果你不使用NH，你就不会这样做。