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The on-find
strategy reads additional columns when the query is invoked. If the query is on-find
optimized, JBoss will execute the following query when the query is executed.
SELECT t0_g.id, t0_g.name, t0_g.nick_name, t0_g.badness FROM gangster t0_g ORDER BY t0_g.id ASC
All of the required data would be in the preload cache, so no additional queries would need to be executed while iterating through the query results. This strategy is effective for queries that return a small amount of data, but it becomes very inefficient when trying to load a large result set into memory. The following table shows the execution of this query:
id | name | nick_name | badness | hangout | organization |
---|---|---|---|---|---|
0 | Yojimbo | Bodyguard | 7 | 0 | Yakuza |
1 | Takeshi | Master | 10 | 1 | Yakuza |
2 | Yuriko | Four finger | 4 | 2 | Yakuza |
3 | Chow | Killer | 9 | 3 | Triads |
4 | Shogi | Lightning | 8 | 4 | Triads |
5 | Valentino | Pizza-Face | 4 | 5 | Mafia |
6 | Toni | Toothless | 2 | 6 | Mafia |
7 | Corleone | Godfather | 6 | 7 | Mafia |
Table 12.2. on-find Optimized Query Execution
The read-ahead
strategy and load-group
for a query is defined in the query
element. If a read-ahead
strategy is not declared in the query
element, the strategy declared in the entity
element or defaults
element is used. The on-find
configuration follows:
<jbosscmp-jdbc> <enterprise-beans> <entity> <ejb-name>GangsterEJB</ejb-name> <!--...--> <query> <query-method> <method-name>findAll_onfind</method-name> <method-params/> </query-method> <jboss-ql><![CDATA[ SELECT OBJECT(g) FROM gangster g ORDER BY g.gangsterId ]]></jboss-ql> <read-ahead> <strategy>on-find</strategy> <page-size>4</page-size> <eager-load-group>basic</eager-load-group> </read-ahead> </query> </entity> </enterprise-beans> </jbosscmp-jdbc>
One problem with the on-find
strategy is that it must load additional data for every entity selected. Commonly in web applications only a fixed number of results are rendered on a page. Since the preloaded data is only valid for the length of the transaction, and a transaction is limited to a single web HTTP hit, most of the preloaded data is not used. The on-load
strategy discussed in the next section does not suffer from this problem.