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From Andrey Razumovsky <razumovsky.and...@gmail.com>
Subject Re: complex query vs performIteratedQuery vs Prefetching
Date Tue, 10 Nov 2009 13:55:48 GMT
Maybe I'm missing something, but i doubt prefetches are implemented for
iterated queries. I would try keeping only setPageSize(1000). It will do all
prefetches and result in same memory payload, because all datarows in your
example are turned into data objects, so they will all be cached in memory.
Smart using query caching would do the trick.
Otherwise, I suggest you forget about data objects in this function of yours
and use only datarows

2009/11/10 Hans Pikkemaat <h.pikkemaat@tsi-solutions.nl>

>  Hi,
>
> I'll give you some insight in what I tried.
>
>         DataContext dataContext = this.createDataContext();
>
>         String sql = "select ... from table1 join table 2 ...";
>         SQLTemplate query = new SQLTemplate(Table1Object.class, sql);
>
> query.addPrefetch(table2.RELATION_PROPERTY).setSemantics(PrefetchTreeNode.JOINT_PREFETCH_SEMANTICS);
>         query.setPageSize(1000);
>
>         ResultIterator iterator = dataContext.performIteratedQuery(query);
>
>         while (iterator.hasNextRow()) {
>             DataRow dataRow = (DataRow) iterator.nextDataRow();
>             Table1Object table1 = (Table1Object)
> dataContext.objectFromDataRow(Table1Object.class, dataRow, false);
>             List<Table2Object> list = table1.getTable2Objects();
>             for (Table2Object t2o:list) {
>                System.out.println(t2o.getName());
>             }
>          }
>
> When the for loop starts and gets the iterator from the list, then a
> separate query is executed, indicating
> that the prefetching didn't work.
>
> tx
>
> Hans
>
>
> Andrey Razumovsky wrote:
>
> Hi,
>
> I think the best in your case would be using disjoint prefetching;http://cayenne.apache.org/doc20/prefetching.html
> This way both table data will be returned altogether (in same db row), so
> you will be able to iterate though them at the same time.
>
> I also highly encourage you to try today-released Cayenne3.0 beta. It proved
> to be stable and many prefetching (ant other!) improvements were made. Check
> the main site!
>
> 2009/11/10 Hans Pikkemaat <h.pikkemaat@tsi-solutions.nl> <h.pikkemaat@tsi-solutions.nl>
>
>    Hi,
>
> My case:
>
> I have a complex query which returns a huge amount of data. It returns data
> from
> two tables which are joined.
>
> Because the amount of data is huge I cannot load all of it into memory.
> For this reason I want to use an iterated query.
>
> I also want to prevent cayenne from executing a query for each detail
> record
> so I want to use prefetching.
>
> What I tried:
>
>   - I'm using cayenne 2.0.4.
>   - I use SQLTemplate to instantiate the query.
>   - I set the page size to 1000 to prevent loading the whole db (required
> for postgres)
>   - I use performIteratedQuery to run the query.
>   - As I get DataRows back from the iterated query I use
> dataContext.objectFromDataRow(..) to create my objects
>
> The Problem:
>
>   The relation is not automatically setup. This means that for every record
> I access the detail table a query is executed.
>
> My Questions:
>
>   - How should I use prefetching in combination with SQLTemplate and an
> iterated query which returns DataRows?
>   -  Is there a way to manually setup this relationship (without changing
> the state of the parent object?)
>   -  Is there another way to do what I want using Cayenne?
>
>
>
>
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> Hans Pikkemaat
>  Java Developer (Services Team)
>  E-mail: h.pikkemaat@tsi-solutions.nl      www.tsi-solutions.nl
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-- 
Andrey

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