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

[Sanne Grinovero]

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