[hibernate-dev] Delay the identity generation to flush-time for transactional contexts too

[Vlad Mihalcea]

I'm sure it require a significant change since the persistence context is
very much tied to the entity identifier.
Theoretically, it could work because the identifier is required for
database operations and for merging as well.

Because the EntityIdentityInsertAction already has the isDelayed property,
maybe we could better reason if we could eventually make this a reasonable
default.
Anyway, it's worth judging if it's worth the development cost and if we
can't afford the change, maybe we could add a Hibernate-native Batching API
to offer an alternative.

Vlad

On Mon, Dec 7, 2015 at 8:04 PM, Sanne Grinovero <sanne at hibernate.org> wrote:

> As soon as a new entity becomes "managed", it needs to be tracked by
> the Persistence Context (the container within each Session)..
> obviously, as otherwise we wouldn't be able to know what needs to be
> eventually flushed.
>
> And within the PC each entity must have an id assigned: it would be
> nice to have a data structure to be able to deal with reference
> equality, but then how would you know when a "merge" operation on a
> different object instance is meant to replace one of the previous
> references?
>
> I wonder if we could handle all corner cases like a merge operation by
> tracking references too (as an alternative to ids), and essentially
> have ids assigned as late as possible.
>
> But I'm pretty sure that we have an API contract that mandates that a
> user can read the Id after the entity has been "saved", even if we
> didn't commit yet, which implies the entities should also be
> instrumented so that a getId() can trigger a lazy initialization of
> the id.
>
> Since this lazy initialization needs to happen on the instances passed
> (and constructed) by the end user, I suspect such a feature would only
> be safe to be enabled for those using bytecode enhanced entities?
>
> Seems like a big one.. you think it's worth it? People might simply
> use an in-memory id generation strategy.
>
> interesting thoughts!
>
> Sanne
>
>
> On 6 December 2015 at 05:46, Vlad Mihalcea <mihalcea.vlad at gmail.com>
> wrote:
> > HI,
> >
> > I always wandered why the identity generator needs to assign an
> identifier
> > prior to flushing.
> > Because of this behavior, the JDBC batching is disabled which is bad news
> > for MySQL where most setups use IDENTITY instead of TABLE generation
> type.
> >
> > Since the AbstractSaveEventListener supports the
> shouldDelayIdentityInserts
> > option for when there is no transaction n progress, we could make this a
> > default anyway.
> >
> > If the inserts are delayed to flush-time, we could batch them based on
> the
> > current dialect. Only the SQL Server dialect and Oracle 11g don't allow
> > fetching the resulting keys from a batch. But Oracle 12c (which supports
> > IDENTITY), PostgreSQL and MySQL can save a batch of inserts and fetch the
> > generated keys.
> >
> > If I judge from a Stack Overflow perspective, most questions are in the
> > context of MySQL, so many Hibernate users use MySQL in fact.
> >
> > What do you think? Should we make this change and allow MySQL users to
> > batch JDBC inserts even for IDENTITY.
> >
> > Vlad
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