Imagine a scenario like: Query is executed but some entities that are part of the resultset are not cached in the second-level cache; the query itself is cached with: query.setCacheable(true);
What Hibernate will do in this case? As the query is already cached, it will just try to load the needed entities. But it will load them with
as many SQL requests as the number of non-cached entities.
This, generally, leads to even worse performance than with non-cached queries. Which, in turn, means that unless we know 80-90% of entities that would appear in the resultset, are cached query caching should not be used, at all! Because, if your query returns 100 resulting objects and only 50 of them are in the second-level cache, your single cached query will result in 50 SQL requests to the database -
terrible!
Now, it is very hard to have the majority of entities in the cache - JVMs have 2G memory limitation, after all. Even if you have entity-caching enabled, as you need to set some LRU rule, bad things, like described above, may happen quite often and actually slow down your system significantly.
I was advised to use batch-loading to help this problem. The idea is that, batch-size attribute in the hbm mapping forces Hibernate to load all missing objects (or a least the number equal to batch-size) in one SQL, rather then each object individually.
This would really help, if I was able to achieve it, but for some reason - I can not make it happen :(
OK, here is what I am trying: for the simplicity, I have entity caching disabled for my entity altogether - to be sure that these entities are not in the second-level cache (caching enabled and entities not in the cache because of LRU is just a flavor of this situation, right?). I have setCachable(true) for my query. I am firing query twice - first time I see that objects are retrieved with one SQL (query is not cached yet) but on the second run - I see, through Hibernate's show_sql, 5 SQLs going to the database. 5 is the number of objects in the resultset.
Here is my mapping:
Code:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping>
<class name="org.digi.Message" table="DG_MESSAGE" batch-size="20" proxy="org.digi.Message">
<composite-id>
<key-property name="key" type="java.lang.String" column="MESSAGE_KEY"/>
<key-property name="locale" type="java.lang.String" column="LANG_ISO" length="2"/>
<key-property name="siteId" type="java.lang.String" column="SITE_ID" length="100"/>
</composite-id>
<property name="message" type="java.lang.String" column="MESSAGE_UTF8"/>
<property name="created" type="java.sql.Timestamp" column="CREATED"/>
</class>
</hibernate-mapping>
as you can see, I have batch-size="20" indicated. In the begining, I had only that but then I added proxy="...", too, as I was told that batch-size only works for proxyable objects (??).
It did not help.Each entity is still loaded with an individual query :(
Any help is appreciated.
P.S. following is the Java code used:
Code:
public static void objQueryList ( Session session ) throws HibernateException{
Query query = session.createQuery(
"select m from Message as m where m.key = :key");
query.setString("key", "login:remember");
query.setCacheable(true);
Message msg = null;
List results = query.list();
for (Iterator it = results.iterator(); it.hasNext(); ) {
msg = new Message();
msg = (Message) it.next();
System.out.println("Message: " + msg.getMessage() );
}
}
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