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Advanced query options
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Usually, you use an ORDER BY with paginated queries, however. Even though you don t need a FROM clause in a collection filter, you may have one if you like. A collection filter doesn t even need to return elements of the collection being filtered. The next query returns any Category with the same name as a category in the given collection:
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String filterString = "select other from Category other where this.name = other.name"; List result = session.createFilter( cat.getChildCategories(), filterString ) .list();
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The following query returns a collection of Users who have bid on the item:
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List result = session.createFilter( item.getBids(), "select this.bidder" ) .list();
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The next query returns all these users bids (including those for other items):
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List result = session.createFilter( item.getBids(), "select elements(this.bidder.bids)" ).list();
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Note that the query uses the special HQL elements() function to project all elements of a collection. All this is a lot of fun, but the most important reason for the existence of collection filters is to allow the application to retrieve some elements of a collection without initializing the whole collection. In the case of large collections, this is important to achieve acceptable performance. The following query retrieves all bids made by a user in the past week:
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List result = session.createFilter( user.getBids(), "where this.created > :oneWeekAgo" ) .setTimestamp("oneWeekAgo", oneWeekAgo) .list();
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Again, this doesn t initialize the bids collection of the User. Queries, no matter in what language and what API they re written, should always be tuned to perform as expected before you decide to speed them up with the optional query cache.
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Caching query results
15.4 Caching query results
We talked about the second-level cache and Hibernate s general cache architecture in chapter 13, section 13.3, Caching fundamentals. You know that the second-level cache is a shared cache of data, and that Hibernate tries to resolve data through a lookup in this cache whenever you access an unloaded proxy or collection or when you load an object by identifier (these are all identifier lookups, from the point of view of the second-level cache). Query results, on the other hand, are by default not cached. Some queries still use the second-level cache, depending on how you execute a query. For example, if you decide to execute a query with iterate(), as we showed in the previous chapter, only the primary keys of entities are retrieved from the database, and entity data is looked up through the first-level and, if enabled for a particular entity, second-level cache. We also concluded that this option makes sense only if the second-level cache is enabled, because an optimization of column reads usually doesn t influence performance. Caching query results is a completely different issue. The query result cache is by default disabled, and every HQL, JPA QL, SQL, and Criteria query always hits the database first. We first show you how to enable the query result cache and how it works. We then discuss why it s disabled and why few queries benefit from result caching.
15.4.1 Enabling the query result cache
The query cache must be enabled using a Hibernate configuration property:
hibernate.cache.use_query_cache = true
However, this setting alone isn t enough for Hibernate to cache query results. By default, all queries always ignore the cache. To enable query caching for a particular query (to allow its results to be added to the cache, and to allow it to draw its results from the cache), you use the org.hibernate.Query interface.
Query categoryByName = session.createQuery("from Category c where c.name = :name"); categoryByName.setString("name", categoryName); categoryByName.setCacheable(true);
The setCachable() method enables the result cache. It s also available on the Criteria API. If you want to enable result caching for a javax.persistence.Query, use setHint("org.hibernate.cacheable", true).
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