2d barcode vb.net Figure 6.1 Logical steps applied during a Hibernate Search query in Java

Making Data Matrix in Java Figure 6.1 Logical steps applied during a Hibernate Search query

Figure 6.1 Logical steps applied during a Hibernate Search query
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Understanding the query paradigm
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use of pagination, the number of elements read can be drastically reduced (and performance improved). As you will see in section 6.4, using pagination limits is recommended from both a performance and a business point of view. Hibernate Search extracts the class and identifier from each Document. These two pieces of information will be necessary to load the object corresponding to the Document from the persistence context. If the query uses projection (see section 6.5), this operation also reads projected properties from the index. Hibernate Search loads the necessary entity instances based on their class and identifier using the Hibernate Session, the persistence context. Various techniques are used to minimize the number of database queries. The retrieved objects are returned in the same order as the Lucene Documents (usually ordered by relevance) and passed to the application as a List, Iterator, or ScrollableResults as an org.hibernate.Query or javax.persistence.Query would do for an HQL query. Hibernate Core provides these objects from the persistence context; lazy loading, transparent navigation, and transparent object management (any change to the object is propagated to the database and the index) are granted.
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Isn t hydrating objects from the Hibernate Session much slower than from Lucene
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Retrieving documents matching a Lucene query consists of two steps:
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Read the index files to find the matching documents and order them by relevance (if you use Lucene s sort capability, this becomes slightly more complicated but is in essence similar). Load the stored fields for each accessed Document.
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Document fields are stored in a different set of files than the index information. Loading a Document is essentially leading to a random file access. Looking up a row from a database by id and retrieving the columns is not fundamentally different from the second step in Lucene. The performance difference between the two techniques more likely depends on additional factors, such as:
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Input/output performance differences between Lucene (usually disk access and random file access) and the database (usually network access) Availability of the data in the database cache
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On top of the raw performance of each operation, don t forget that Hibernate caches objects in the persistence context (and if set up, in the second-level cache), which in some situations eliminates database round-trips. Beyond performance, the level of features available to objects loaded from the persistence context (lazy loading, managed objects, unicity, and so on) negates the performance consideration most of the time. Hibernate Search also provides a projection mode that lets you bypass the persistence context if needed. We ll describe this mode in greater detail in section 6.5.
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Querying with Hibernate Search
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Hibernate Search queries have one additional benefit. If the index is slightly out of date compared to the database, some documents can point to objects that are no longer present in the database. Index desynchronization can come from the small replication delay occurring in a cluster when asynchronous replication is used (see chapter 10) or because some third-party applications have updated the database and the change has not yet been reflected. Hibernate Search transparently removes those orphaned documents from the results, and this inconsistency is hidden to the application. Enough theory for now; let s see how to build a Hibernate Search query!
Building a Hibernate Search query
A Hibernate Search query is essentially a smart wrapper around a Lucene query. The first step for an application developer is to write the Lucene query that will retrieve the necessary objects. We ll cover this vast topic in chapter 7. The next step is to build a Hibernate Search query. Like most operations specific to Hibernate Search, creating a full-text query is done through the FullTextSession or FullTextEntityManager API.
Building a FullTextSession or a FullTextEntityManager
Hibernate Search queries are created from a FullTextSession object, just like a Hibernate HQL query is created from a Session object. FullTextSession is an interface that subclasses org.hibernate.Session. It is quite transparent to replace Session references by FullTextSession references in a code base. The FullTextSession implementation is a wrapper around a Session object, which can be built using the org.hibernate.search.Search.getFullTextSession method, as shown in listing 6.2.
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