It is always recommended to select only those columns which are required for our use case. If you are using the CriteriaQuery, use projections to select required columns. Fetching the entire entity would degrade the application's performance when the table has too many columns, so the database needs to go through each block of the stored page to retrieve them, and we might not even need all of those columns in our use case. Also, if we are using an entity instead of the DTO class, persistence context has to manage the entities and also fetches associated/child entities when required. This adds an overhead. Instead of fetching  the entire entity, fetch only the required columns:

SELECT a FROM Account a WHERE a.accountId= 123456;

Fetch a specific column as follows:

SELECT a.accountId, a.name FROM Account a WHERE a.accountId = 123456;

The better way to use query-specific fetching is to use DTO projection. Our entities are managed by a persistence context. So, it would be easier to fetch ResultSet to the entity, in case we want to update it. We set the new values to the setter methods, and Hibernate will take care of the SQL statements to update it. This easiness comes with the price of performance since Hibernate needs to do dirty checks on all managed entities to find out if it needs to save any changes to the database. DTO are the POJO classes which are the same as our entities, however, it's not managed by persistence.

We can fetch specific columns in JPQL by using the constructor expression, as follows:

entityManager.createQuery("SELECT new com.packt.springhighperformance.ch6.bankingapp.dto.AccountDto(a.id, a.name) FROM Account a").getResultList();

Similarly, we can do the same thing by using CriteriaQuery and JPAMetamodel, as follows:

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery q = cb.createQuery(AccountDTO.class);
Root root = q.from(Account.class);
q.select(cb.construct(AccountDTO.class, root.get(Account_.accountNumber), root.get(Account_.name)));

List authors = em.createQuery(q).getResultList();