Why Does NHibernate QueryOver with Fetch Cause Multiple Database Queries, and How Can I Optimize It?

NHibernate QueryOver with Fetch Incurring Multiple Query Executions

Executing a QueryOver with a Fetch clause to retrieve a parent entity and its child collection often results in excessive database hits. This behavior occurs because NHibernate issues a separate query for each row in the child collection.

To understand this, consider a scenario with a UserRole entity and a UsersInRole collection. The first query retrieves the UserRole entities:

SELECT ... FROM UserRoles
left outer join UsersInRoles on ...

For each UserRole retrieved, NHibernate then executes a separate query to fetch the UsersInRole collection, resulting in numerous queries in the format:

SELECT ... FROM UsersInRoles
left outer join UserRoles on ...
WHERE UserRoles.UserId=?

To optimize this behavior, NHibernate allows setting a BatchSize property on the HasManyToMany mapping:

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

By specifying this option, NHibernate will load the child collection in batches, reducing the number of database queries to a maximum of 1 (batch-size / page-size). This ensures efficient retrieval of related data while avoiding excessive database hits.

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