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

[Gail Badner]

Steve, is this related to https://hibernate.atlassian.net/browse/HHH-3949?

On Thu, Mar 1, 2018 at 10:11 PM, Gail Badner <gbadner at redhat.com> wrote:

> 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/hibern
>> ate/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_s
>> ingle/Hibernate_User_Guide.html#fetching-fetchmode-join
>>
>
>