[hibernate-dev] HHH-11223 : Initialize bytecode enhanced lazy group using a single SQL statement

[Gail Badner]

This is not necessarily needed for 5.3.0, but I would like to get it into
the 5.3 branch.

So far, there is one change to an SPI, Queryable#lazyGroupSelectFragment is
added:

public String lazyGroupSelectFragment(
String tableAlias,
String suffix,
String fetchGroupName);

The method gets implemented in AbstractEntityPersister.

I don't think it's possible to create a default method in Queryable because
the method needs access to private/protect methods in
AbstractEntityPersister.

If I don't get HHH-11223 in for 5.3.0, should I at least push the SPI
change for 5.3.0?

On Thu, Mar 1, 2018 at 9:55 PM, Gail Badner <gbadner at redhat.com> wrote:

> Hi,
>
> Currently, when a lazy group is loaded that has associations, there are 2
> SQL statements executed.
>
> For example, suppose the DEFAULT lazy group contains the following:
>
> * Order#customer is a lazy many-to-one association;
>
> * Order#text is a lazy LOB.
>
>
> When one of these properties is referenced, Hibernate loads both of them.
>
>
> Currently, it takes 2 SQL statements to get the association loaded.
>
>
> 1) Loads #text and the FK value for #customer (select o.text,
> o.customer_id from orders o where o.id = ?)
>
> 2) Loads the state for #customer (select ... from customers c where c.id =
> ?)
>
>
> In other words, FetchMode.SELECT is used to load associations in a lazy
> group.
>
>
> HHH-11223 involves combining the 2 SQL statements into a single statement
> using a join (i.e., using FetchMode.JOIN), but only when at least one of
> the following is true:
>
>
> * caching is disabled for the SessionFactory (e.g.,
> hibernate.cache.use_second_level_cache=false;
>
> * caching is not enabled for "get" operations (i.e.,
> session.getCacheMode().isGetEnabled() == false);
>
> * the associated entity type (and subclasses?) is not cached
>
> HHH-11223 only mentions to-one associations, but I think collections in a
> lazy group could also be loaded using a join as well.
>
> I think I'm pretty close, but I need to do a lot more testing to ensure it
> covers things like:
>
> * lazy association in composite ID
> * lazy association in an embeddable
> * lazy association in a secondary table
> * lazy association in a subclass
> * too many bags
>
> This is what I have implemented so far: https://github.com/
> hibernate/hibernate-orm/pull/2170
>
> At the moment, I'm not looking for a comprehensive review. I'm just
> looking for a review of the overall design.
>
> I don't want to get into the nitty-gritty here, but, basically, I've
> created a new type of AbstractLoadPlanBasedEntityLoader, LazyGroupLoader,
> which loads the lazy group properties into the existing entity.
>
> If caching is disabled for the SessionFactory, then one loader is
> constructed per lazy group, and associations will be loaded using
> FetchMode.JOIN.
>
> If caching is enabled for the SessionFactory, then 2 loaders are
> constructed per lazy group. One loader is used when caching is enabled for
> "get" operations (using Fetch.SELECT for cached associated
> entities/collections; Fetch.JOIN for other associations). The other loader
> is used when caching is disabled for "get" operations.
>
> Here are some of the changes to the default behavior (described above)
> that could be useful:
>
> 1) Change all associations in lazy groups to be loaded using
> FetchMode.SELECT (as is currently done without the fix), or using
> FetchMode.JOIN (regardless of caching). A new configuration property could
> be added:
>
> hibernate.bytecode.lazyGroup_association_fetchMode=(default | select |
> join )
>
> 2) Allow the FetchMode to be configured for a particular lazy association.
> According to the documentation [1], @Fetch can't be used for this because,
> "... FetchMode.JOIN acts as a FetchType.EAGER strategy. Even if we mark the
> association as FetchType.LAZY, the FetchMode.JOIN will load the association
> eagerly."
>
> A new annotation could be introduced: @LazyFetch.
>
> Comments or suggestions?
>
> Thanks,
> Gail
>
> [1] http://docs.jboss.org/hibernate/orm/5.2/userguide/html_
> single/Hibernate_User_Guide.html#fetching-fetchmode-join
>