NHibernate Lazy=“Extra”

Question

Is there a good explanation out there on what exactly lazy=\"extra\" is capable of?

All the posts I\'ve seen all just repeat the fact that it turns references to

Answer 1

The lazy = extra allow to count the element of a collection without needing of fetching it, since the lazy entity is decorated with a proxy, when the client code ask for the .Count on the collection, a proper "select count" query is issued to the database. Without lazy=extra the collection is read from the database.

Answer 2

for version 2.x it is only used to translate a collection.Count() into a select count and as far as i can see in the source, it will also allow the construct collection[5] to fetch that particular entity (with index 5) instead of hydrating the whole collection.

For version 3.x i didn't see anything related in the release notes

Answer 3

Just tried calling Any() on a Collection Customer.Orders mapped with lazy="extra"

customer.Orders.Any()

and the resulting SQL statement looked something like this (simplified):

SELECT *
FROM Order
WHERE CustomerId = 120

Whereas when calling

customer.Orders.Count > 0

the resulting SQL looked like this:

SELECT count(*)
FROM Order
WHERE CustomerId = 120