Why Does NHibernate QueryOver with Eager Fetching Generate Multiple SQL Queries?

NHibernate QueryOver with Fetch Resulting in Multiple SQL Queries

Query Issue:

When using QueryOver to fetch a related list using eager loading, multiple database hits occur, resulting in numerous separate SQL queries.

Example:

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

Explanation:

This behavior is due to the nature of many-to-many relationships. In the example mapping, a UserRole entity has a many-to-many relationship with UsersInRole entities. When Eager loading is used, NHibernate attempts to load all related UsersInRole entities for each retrieved UserRole entity.

However, since the session may not yet contain all related entities, multiple SQL queries are issued to retrieve these relationships. The initial query retrieves the UserRole entities, and subsequent queries are issued for each UserRole to retrieve its related UsersInRole entities.

Solution:

One effective solution to minimize SQL queries is to use batch fetching. By setting the BatchSize property on the HasManyToMany mapping, NHibernate can fetch multiple related entities in a single SQL query.

HasManyToMany(x => x.UsersInRole)
...
.BatchSize(25)  

By setting the batch size to an appropriate value (e.g., 25), NHibernate will issue a limited number of SQL queries, reducing the total database hits and improving performance.

Additionally, setting the batch size on the Class map can also help optimize the overall fetching strategy.

The above is the detailed content of Why Does NHibernate QueryOver with Eager Fetching Generate Multiple SQL Queries?. For more information, please follow other related articles on the PHP Chinese website!