Joins, reporting queries, and subselects (perhaps also the (Web design)
Joins, reporting queries, and subselects (perhaps also the second-level cache) and manage uniqueness, and so on. If you ever wish to avoid the overhead of managing the persistence context cache, report queries give you a way to do this. The overhead of a Hibernate report query compared to direct SQL/JDBC isn t usually measurable, even in unrealistic extreme cases, like loading one million objects from a local database without network latency. Report queries using projection in HQL and JPA QL let you specify which prop erties you wish to retrieve. For report queries, you aren t selecting entities in man aged state, but only properties or aggregated values: select user.lastname, count(user) from User user group by user.lastname This query doesn t return persistent entity instances, so Hibernate doesn t add any persistent object to the persistence context cache. This means that no object must be watched for dirty state either. Therefore, reporting queries result in faster release of allocated memory, because objects aren t kept in the persistence context cache until the context is closed they may be garbage collected as soon as they re dereferenced by the application, after executing the report. Almost always, these considerations are extremely minor, so don t go out and rewrite all your read-only transactions to use report queries instead of transactional, cached, and managed objects. Report queries are more verbose and (arguably) less object-oriented. They also make less efficient use of Hibernate s caches, which is much more important once you consider the overhead of remote communication with the database in production systems. You should wait until you find a case where you have a real performance problem before using this optimization. You can already create really complex HQL and JPA QL queries with what you ve seen so far. Even more advanced queries may include nested statements, known as subselects. 14.3.3 Using subselects An important and powerful feature of SQL is subselects. A subselect is a select query embedded in another query, usually in the SELECT, FROM, or WHERE clauses. HQL and JPA QL support subqueries in the WHEREclause. Subselects in the FROM clause aren t supported by HQL and JPA QL (although the specification lists them as a possible future extension) because both languages have no transitive closure. The result of a query may not be tabular, so it can t be reused for selection in a
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