Why Does NHibernate QueryOver's Eager Loading Cause Multiple Database Queries?

Eager Loading in NHibernate QueryOver Results in Multiple Database Queries

When performing an eager fetch of a related list using NHibernate QueryOver, it may result in multiple database queries. This can lead to significantly increased database traffic and reduced performance.

Consider the following scenario:

Session.QueryOver<UserRole>()
       .Fetch(x => x.UsersInRole).Eager
       .List();

This query attempts to select a collection of UserRoles and eagerly fetch the related UsersInRole. However, this results in multiple separate database queries:

• An initial query to retrieve all UserRoles.
• Additional queries for each UserRole to retrieve the associated UsersInRole.

Cause of Multiple Queries

NHibernate uses batch fetching to retrieve related entities efficiently. However, when the number of related entities exceeds the batch size, additional queries are required. In the above scenario, the batch size is likely too small to retrieve all the UsersInRole in one query.

Solution: Adjust Batch Size

To address this issue, you can adjust the batch size used for eager fetching. By setting a larger batch size, NHibernate will attempt to fetch more related entities in a single query. This can significantly reduce the number of database hits.

Modify the mapping as follows:

public class UserRoleMap : ClassMap<UserRole>
{
    public UserRoleMap()
    {
        ...
        HasManyToMany(x => x.UsersInRole)
        ...
        .BatchSize(25);
    }
}

Setting the batch size to 25 will instruct NHibernate to fetch a maximum of 25 UsersInRole entities per query. This may result in fewer queries and improved performance.

Additional Considerations

• Using fetch="fetch" in HQL can also result in multiple database queries. To avoid this, use fetch="select" instead.
• Eager fetching can significantly increase memory usage. Use it judiciously and only when necessary.

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