Eager fetching is the ability to efficiently load subclass data and
related objects along with the base instances being queried.
Typically, Kodo has to make a trip to the database whenever a
relation is loaded, or when you first access data that is mapped to a
table other than the least-derived superclass table. If you perform a
query that returns 100
Person objects, and then
you have to retrieve the
Address for each
person, Kodo may make as many as 101 queries (the initial
query, plus one for the address of each person returned). Or if some
Person instances turn out to be
has additional data in its own joined table, Kodo once again might need
to make extra database trips to access the additional employee data.
With eager fetching, Kodo can reduce these cases to a single query.
Eager fetching only affects relations in the active fetch groups, and is limited by the declared maximum fetch depth and field recursion depth (see Section 5.6, “Fetch Groups”). In other words, relations that would not normally be loaded immediately when retrieving an object or accessing a field are not affected by eager fetching. In our example above, the address of each person would only be eagerly fetched if the query were configured to include the address field or its fetch group, or if the address were in the default fetch group. This allows you to control exactly which fields are eagerly fetched in different situations. Similarly, queries that exclude subclasses aren't affected by eager subclass fetching, described below.
Eager fetching has three modes:
none: No eager fetching is performed.
Related objects are always loaded in an independent select
statement. No joined subclass data is loaded unless it is in
the table(s) for the base type being queried. Unjoined subclass
data is loaded using separate select statements rather than
a SQL UNION operation.
join: In this mode, Kodo joins to to-one
relations in the configured fetch groups. If Kodo is loading
data for a single instance, then Kodo will also
join to any collection field in the configured
fetch groups. When loading data for multiple instances, though,
(such as when executing a
will not join to collections by default. Instead, Kodo defaults
parallel mode for collections, as
described below. You can force Kodo use a join rather than
parallel mode for a collection field using the metadata
extension described in Section 126.96.36.199, “Eager Fetch Mode”.
join mode, Kodo uses a left outer join
(or inner join, if the relations' field metadata declares the
relation non-nullable) to select the
related data along with the data for the target objects.
This process works recursively for to-one joins, so that if
Person has an
Address has a
TelephoneNumber, and the fetch groups
are configured correctly, Kodo might issue a single select that
joins across the tables for all three classes. To-many joins
can not recursively spawn other to-many joins, but they can
spawn recursive to-one joins.
join subclass fetch mode, subclass
data in joined tables is selected by outer joining to all
possible subclass tables of the type being queried. Unjoined
subclass data is selected with a SQL UNION where possible.
As you'll see below, subclass data fetching is configured
separately from relation fetching, and can be disabled for
parallel: Under this mode, Kodo selects
to-one relations and joined collections as outlined
join mode description above. Unjoined
collection fields, however, are eagerly fetched using a
separate select statement for each collection, executed in
parallel with the select statement for the target objects.
The parallel selects use the
conditions from the primary select, but add their own joins to
reach the related data. Thus, if you perform a query that
Company objects, where each
company has a list of
Department objects, Kodo will make
3 queries. The first will select the company objects, the second
will select the employees for those companies, and
the third will select the departments for the same companies.
Just as for joins, this process can be
recursively applied to the objects in the relations being
eagerly fetched. Continuing our example, if the
had a list of
Projects in one of the
fetch groups being loaded, Kodo would execute a single
additional select in parallel to load the projects of all
employees of the matching companies.
Using an additional select to load each collection avoids transferring more data than necessary from the database to the application. If eager joins were used instead of parallel select statements, each collection added to the configured fetch groups would cause the amount of data being transferred to rise dangerously, to the point that you could easily overwhelm the network.
Polymorphic to-one relations to table-per-class mappings use parallel eager fetching because proper joins are impossible. You can force other to-one relations to use parallel rather than join mode eager fetching using the metadata extension described in Section 188.8.131.52, “Eager Fetch Mode”.
Setting your subclass fetch mode to
affects table-per-class and vertical inheritance hierarchies.
Under parallel mode, Kodo issues separate selects for each
subclass in a table-per-class inheritance hierarchy, rather
than UNIONing all subclass tables together as in join mode.
This applies to any operation on a table-per-class base class:
query, by-id lookup, or relation traversal.
When dealing with a vertically-mapped hierarchy, on the other hand, parallel subclass fetch mode only applies to queries. Rather than outer-joining to subclass tables, Kodo will issue the query separately for each subclass. In all other situations, parallel subclass fetch mode acts just like join mode in regards to vertically-mapped subclasses.
When Kodo knows that it is selecting for a single object only,
it never uses
parallel mode, because the
additional selects can be made lazily just as efficiently.
This mode only increases efficiency over
mode when multiple objects with eager relations
are being loaded, or when multiple selects might be faster than
joining to all possible subclasses.
You can control Kodo's default eager fetch mode through the
properties. Set each of these properties to one of the mode names
described in the previous section:
parallel. If left unset, the eager fetch mode defaults
parallel and the subclass fetch mode defaults
join. These are generally the most robust and
You can easily override the default fetch modes at runtime for any lookup or query through Kodo's fetch configuration APIs. See Chapter 9, Runtime Extensions for details.
Example 5.20. Setting the Default Eager Fetch Mode
JPA XML format:
<property name="kodo.jdbc.EagerFetchMode" value="parallel"/> <property name="kodo.jdbc.SubclassFetchMode" value="join"/>
JDO properties format:
kodo.jdbc.EagerFetchMode: parallel kodo.jdbc.SubclassFetchMode: join
Example 5.21. Setting the Eager Fetch Mode at Runtime
import org.apache.openjpa.persistence.*; import org.apache.openjpa.persistence.jdbc.*; ... Query q = em.createQuery ("select p from Person p where p.address.state = 'TX'"); OpenJPAQuery oq = OpenJPAPersistence.cast (q); JDBCFetchPlan fetch = (JDBCFetchPlan) oq.getFetchPlan (); fetch.setEagerFetchMode (JDBCFetchPlan.EAGER_PARALLEL); fetch.setSubclassFetchMode (JDBCFetchPlan.EAGER_JOIN); List results = q.getResultList ();
import kodo.jdo.jdbc.*; ... Query q = pm.newQuery (Person.class, "address.state == 'TX'"); JDBCFetchPlan fetch = (JDBCFetchPlan) q.getFetchPlan (); fetch.setEagerFetchMode (JDBCFetchPlan.EAGER_PARALLEL); fetch.setSubclassFetchMode (JDBCFetchPlan.EAGER_JOIN); List results = (List) q.execute ();
You can specify a default subclass fetch mode for an individual
class with the metadata extension described in
Section 184.108.40.206, “Subclass Fetch Mode”.
Note, however, that you cannot "upgrade" the runtime fetch mode
with your class setting. If the runtime fetch mode is
none, no eager subclass data fetching will
take place, regardless of your metadata setting.
This applies to the eager fetch mode metadata extension as well
(see Section 220.127.116.11, “Eager Fetch Mode”).
You can use this extension to disable eager fetching on a field or
to declare that a collection would rather use joins than parallel
selects or vice versa. But an extension value of
join won't cause any eager joining if the fetch
configuration's setting is
There are several important points that you should consider when using eager fetching:
When you are using
parallel eager fetch
mode and you have large result sets enabled (see
Section 4.11, “Large Result Sets”) or you place
a range on a query, Kodo performs the needed parallel
selects on one page of results at a time. For example,
FetchBatchSize is set to
20, and you perform a large result set
query on a class that has collection fields in the
configured fetch groups. Kodo will immediately cache
20 results of the query using
join mode eager fetching only. Then,
it will issue the extra selects needed to eager fetch
your collection fields according to
mode. Each select will use a SQL
IN clause (or multiple
clauses if your class has a compound primary key)
to limit the selected collection elements to those owned
by the 20 cached results.
Once you iterate past the first 20 results, Kodo will cache the next 20 and again issue any needed extra selects for collection fields, and so on. This pattern ensures that you get the benefits of eager fetching without bringing more data into memory than anticipated.
Once Kodo eager-joins into a class, it cannot issue any further eager to-many joins or parallel selects from that class in the same query. To-one joins, however, can recurse to any level.
Using a to-many join makes it impossible to determine the number of instances the result set contains without traversing the entire set. This is because each result object might be represented by multiple rows. Thus, queries with a range specification or queries configured for lazy result set traversal automatically turn off eager to-many joining.
Kodo cannot eagerly join to polymorphic relations to non-leaf classes in a table-per-class inheritance hierarchy. You can work around this restriction using the mapping extensions described in Section 18.104.22.168, “Nonpolymorphic”.