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7.6.2 Using iterate() queries
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The iterate() method of the Session and Query interfaces behaves differently than the find() and list() methods. It s provided specifically to let you take full advantage of the second-level cache. Consider the following code:
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Query categoryByName = session.createQuery("from Category c where c.name like :name"); categoryByName.setString("name", categoryNamePattern); List categories = categoryByName.list();
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Retrieving objects efficiently
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This query results in execution of an SQL select, with all columns of the CATEGORY table included in the select clause:
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select CATEGORY_ID, NAME, PARENT_ID from CATEGORY where NAME like
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If we expect that categories are already cached in the session or second-level cache, then we only need the identifier value (the key to the cache). This will reduce the amount of data we have to fetch from the database. The following SQL would be slightly more efficient:
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select CATEGORY_ID from CATEGORY where NAME like
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We can use the iterate() method:
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Query categoryByName = session.createQuery("from Category c where c.name like :name"); categoryByName.setString("name", categoryNamePattern); Iterator categories = categoryByName.iterate();
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The initial query only retrieves the category primary key values. We then iterate through the result, and Hibernate looks up each Category in the current session and in the second-level cache. If a cache miss occurs, Hibernate executes an additional select, retrieving the category by its primary key from the database. In most cases, this is a minor optimization. It s usually much more important to minimize row reads than to minimize column reads. Still, if your object has large string fields, this technique may be useful to minimize data packets on the network and, therefore, latency. Let s talk about another optimization, which also isn t applicable in every case. So far, we ve only discussed caching the results of a lookup by identifier (including implicit lookups, such as loading a lazy association) in chapter 5. It s also possible to cache the results of Hibernate queries.
7.6.3 Caching queries
For applications that perform many queries and few inserts, deletes, or updates, caching queries can have an impact on performance. However, if the application performs many writes, the query cache won t be utilized efficiently. Hibernate expires a cached query result set when there is any insert, update, or delete of any row of a table that appears in the query. Just as not all classes or collections should be cached, not all queries should be cached or will benefit from caching. For example, if a search screen has many different search criteria, then it s unlikely that the user will choose the same criterion twice. In this case, the cached query results won t be utilized, and we d be better off not enabling caching for that query.
Optimizing object retrieval
Note that the query cache does not cache the entities returned in the query result set, just the identifier values. Hibernate will, however, fully cache the value typed data returned by a projection query. For example, the projection query "select u, b.created from User u, Bid b where b.bidder = u" will result in caching of the identifiers of the users and the date object when they made their bids. It s the responsibility of the second-level cache (in conjunction with the session cache) to cache the actual state of entities. So, if the cached query you just saw is executed again, Hibernate will have the bid-creation dates in the query cache but perform a lookup in the session and second-level cache (or even execute SQL again) for each user that was in the result. This is similar to the lookup strategy of iterate(), as explained in the previous section. The query cache must be enabled using a Hibernate property setting:
hibernate.cache.use_query_cache true
However, this setting alone isn t enough for Hibernate to cache query results. By default, Hibernate 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 Query interface:
Query categoryByName = session.createQuery("from Category c where c.name = :name"); categoryByName.setString("name", categoryName); categoryByName.setCacheable(true);
Even this doesn t give you sufficient granularity, however. Different queries may require different query expiration policies. Hibernate allows you to specify a different named cache region for each query:
Query userByName = session.createQuery("from User u where u.username= :uname"); userByName.setString("uname", username); userByName.setCacheable(true); userByName.setCacheRegion("UserQueries");
You can now configure the cache expiration policies using the region name. When query caching is enabled, the cache regions are as follows:
The default query cache region, net.sf.hibernate.cache.QueryCache Each named region The timestamp cache, net.sf.hibernate.cache.UpdateTimestampsCache, which is a special region that holds timestamps of the most recent updates to each table
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