Web hosting mysql - Defining the global fetch plan persistent object network
Defining the global fetch plan persistent object network should be retrieved and how it should be retrieved. Your goal is to find the best retrieval method and fetching strategy for every use case in your application; at the same time, you also want to minimize the number of SQL queries for best performance. Before we look at the fetch plan options and fetching strategies, we ll give you an overview of the retrieval methods. (We also mention the Hibernate caching system sometimes, but we fully explore it later in this chapter.) You saw how objects are retrieved by identifier earlier in the previous chapter, so we won t repeat it here. Let s go straight to the more flexible query options, HQL (equivalent to JPA QL) and Criteria. Both allow you to create arbitrary queries. The Hibernate Query Language and JPA QL The Hibernate Query Language is an object-oriented dialect of the familiar database query language SQL. HQL bears some close resemblance to ODMG OQL, but unlike OQL, it s adapted for use with SQL databases and is easier to learn (thanks to its close resemblance to SQL) and fully implemented (we don t know of any OQL implementation that is complete). The EJB 3.0 standard defines the Java Persistence query language. This new JPA QL and the HQL have been aligned so that JPA QL is a subset of HQL. A valid JPA QL query is always also a valid HQL query; HQL has more options that should be considered vendor extensions of the standardized subset. HQL is commonly used for object retrieval, not for updating, inserting, or deleting data. Object state synchronization is the job of the persistence manager, not the developer. But, as we ve shown in the previous chapter, HQL and JPA QL support direct bulk operations for updating, deleting, and inserting, if required by the use case (mass data operations). Most of the time, you only need to retrieve objects of a particular class and restrict by the properties of that class. For example, the following query retrieves a user by first name. Query q = session.createQuery( “from User as u where u.firstname = :fname” ); q.setString(”fname”, “John”); List result = q.list(); After preparing query q, you bind a value to the named parameter :fname. The result is returned as a List of User objects.
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