Entity graphs are templates for a particular Persistence query or
operation. They are used when creating fetch plans, or groups of
persistent fields that are retrieved at the same time. Application
developers use fetch plans to group together related persistent fields
to improve runtime performance.
By default, entity fields or properties are fetched lazily. Developers
specify fields or properties as part of a fetch plan, and the
persistence provider will fetch them eagerly.
For example, an email application that stores messages as
entities prioritizes fetching some fields over others. The sender,
subject, and date will be viewed the most often, in mailbox views and
when the message is displayed. The
EmailMessage entity has a
collection of related
EmailAttachment entities. For performance
reasons the attachments should not be fetched until they are needed, but
the file names of the attachment are important. A developer working on
this application might make a fetch plan that eagerly fetches the
important fields from
fetching the lower priority data lazily.