EclipseLink/UserGuide/JPA/Advanced JPA Development/Performance/Fetch Groups
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By default, when you execute an object-level read query for a particular object class, EclipseLink returns all the persistent attributes mapped in the object's descriptor. With this single query, all the object's persistent attributes are defined, and calling their get methods returns the value directly from the object.
When you are interested in only some of the attributes of an object, it may be more efficient to return only a subset of the object's attributes using a fetch group.
Using a fetch group, you can define a subset of an object's attributes and associate the fetch group with either a ReadObjectQuery or ReadAllQuery query. When you execute the query, EclipseLink retrieves only the attributes in the fetch group. EclipseLink automatically executes a query to fetch all the attributes excluded from this subset when and if you call a get method on any one of the excluded attributes.
You can define more than one fetch group for a class. You can optionally designate at most one such fetch group as the default fetch group. If you execute either a ReadObjectQuery or ReadAllQuery query without specifying a fetch group, EclipseLink will use the default fetch group, unless you configure the query otherwise).
You can use fetch groups in JPA projects for EJB objects, as well as for POJO classes. For POJO classes, use partial object querying).
Before using fetch groups, we recommend that you perform a careful analysis of system use. In many cases, the extra queries required to load attributes not in the fetch group could well offset the gain from the partial attribute loading..
In the 2.1.0, EclipseLink introduces the AttributeGroup that can be used configure the use of partial entities in fetch, load, copy, and merge operations.