Hibernate can make efficient use of batch fetching, that is, Hibernate can load several uninitialized proxies if one proxy is accessed (or collections. Batch fetching is an optimization of the lazy select fetching strategy. There are two ways you can tune batch fetching: on the class and the collection level.

Batch fetching for classes/entities is easier to understand. Imagine you have the following situation at runtime: You have 25 Cat instances loaded in a Session, each Cat has a reference to its owner, a Person. The Person class is mapped with a proxy, lazy="true". If you now iterate through all cats and call getOwner() on each, Hibernate will by default execute 25 SELECT statements, to retrieve the proxied owners. You can tune this behavior by specifying a batch-size in the mapping of Person:

<class name="Person" batch-size="10">...</class>

Hibernate will now execute only three queries, the pattern is 10, 10, 5.

You may also enable batch fetching of collections. For example, if each Person has a lazy collection of Cat s, and 10 persons are currently loaded in the Sesssion, iterating through all persons will generate 10 SELECT s, one for every call to getCats(). If you enable batch fetching for the cats collection in the mapping of Person, Hibernate can pre-fetch collections:

<class name="Person">
    <set name="cats" batch-size="3">
        ...
    </set>
</class>

With a batch-size of 8, Hibernate will load 3, 3, 3, 1 collections in four SELECT s. Again, the value of the attribute depends on the expected number of uninitialized collections in a particular Session.

Batch fetching of collections is particularly useful if you have a nested tree of items, ie. the typical bill-of-materials pattern. (Although a nested set or a materialized path might be a better option for read-mostly trees.)