[hibernate-dev] Correlating Java 1.8 Streams and ScrollableResults with database cursors

Steve Ebersole steve at hibernate.org
Mon Nov 21 13:17:30 EST 2016


I had a discussion about this with Vlad on the PR and privately on HipChat.

I personally am -1 on this as well.  As you say Sanne the problem is "if
you all think Hibernate should be able to figure this out
automatically"...  You would never be able to figure this out across all
situations.


On Mon, Nov 21, 2016 at 12:12 PM Sanne Grinovero <sanne at hibernate.org>
wrote:

> Hi Vlad,
> I'm not sure if we should set the fetch size automatically, especially
> not globally. Could we not use the API wisely to encourage people to
> provide an estimate of a reasonable size, which will be different on
> each query?
>
> I'm aware that MySQL requires this strange value to avoid pre-loading
> it all in memory; in fact for example in Hibernate Search the "mass
> index rebuild" has to scroll on the whole dataset, so the
> documentation recommends to set the fetch size..
>
> However in some cases the optimisation might make sense; for example
> if the resultset is small, why would you use pagination when you can
> load it all in one single roundtrip?
>
> On the other hand, if you all think Hibernate should be able to figure
> this out automatically, then maybe I should also change Search to
> automatically set MIN_VALUE when running on MySQL, etc..
>
> Thanks,
> Sanne
>
>
>
>
> On 19 November 2016 at 10:04, Vlad Mihalcea <mihalcea.vlad at gmail.com>
> wrote:
> > Hi,
> >
> > I've noticed this new Jira ticket:
> >
> > https://hibernate.atlassian.net/browse/HHH-11260
> >
> > and I think we should correlate the Session#stream and Session#scroll
> with
> > database cursors.
> >
> > While Oracle uses a fetch size of 10, and SQL Server uses an adaptive
> > cursor, PostgreSQL and MySQL just fetch the whole result set into the
> > client.
> >
> > So, for MySQL we could do as suggested in the Jira ticket, by setting the
> > fetch size to Integer.MIN_VALUE.
> >
> > My question is about PotgreSQL. Postgres requires setting the fetch size
> to
> > some positive value in order to turn the current statement into a
> database
> > cursor.
> > Now, should we set a default value for streaming (e.g. 10), which can be
> > controlled via a new config property:
> >
> > hibernate.jdbc.stream.fetch_size
> >
> > Let me know what you think.
> >
> > Vlad
> > _______________________________________________
> > hibernate-dev mailing list
> > hibernate-dev at lists.jboss.org
> > https://lists.jboss.org/mailman/listinfo/hibernate-dev
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