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From "Mike Kienenberger" <>
Subject Re: localObject vs prefetches: would there be a benefit ?
Date Fri, 02 Nov 2007 13:23:35 GMT
I am no expert in this subject, but the cost of doing I/O is always
going to exceed the cost of in-system memory copies, probably by
orders of magnitude.   The only place where this might not be true is
using an in-memory database such as an HSQLDB mem jdbc connection.

So unless you're copying orders-of-magnitude more data than you'd be
fetching, always copy.

Remember too that a localObject only copies the object, not any of its
relationships.   And the other objects are only pulled in if you
access them, which is even more fine-grained than prefetching.

So my guess is that localObject is ALWAYS orders of magnitude faster.
I suspect it'd be extremely difficult to define a situation where this
was not the case.

On 11/1/07, Marcin Skladaniec <> wrote:
> Hi
> We are improving the performance of our client application (ROP). Not
> surprisingly we have found that adding prefetches did improve the
> speed significantly.
> I have a question though about which is about efficiency of
> localObject. We are fetching a list of objects (query with prefetches)
> to a specific one non-editable context. If the object is to be edited
> it has to be copied to another context which allows committing
> changes. After the object is copied all the related object are
> accessed, so the performance of the copying came to my mind.
> The test I have looks like this:
> CayenneContext context1;
> CayenneContext context2;
> SelectQuery q = new SelectQuery(Painting.class);
> q.addPrefetch(Painting.GALLERY_PROPERTY);
> q.addPrefetch(Painting. GALLERY_PROPERTY + "." + Gallery.CITY_PROPERTY);
> List l = context1.performQuery(q);
> //now the context1 contain all the records I wanted it to contain
> Painting p1 = (Painting) l.get(0);
> Painting p2 = (Painting) context2.localObject(p1.getObjectId(), null);
> //now the context2 contain only the single record I copied
> p2.getGallery().getCity();
> //now  context2 contains the same objects as context1
> All is very quick, but my question is how it will scale when the
> relationship would be to-many and there will be hundreds related
> records.
> What do you think would be a threshold number of related objects which
> have to be localised over which it would be worth doing a new,
> specific select query for that single object (with prefetches). Would
> there be advantage of doing that at all ?
> We are using cutting edge version of cayenne (I think we use build
> about 2-3 weeks old).
> Marcin

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