Re: complex query vs performIteratedQuery vs Prefetching

Tue, 10 Nov 2009 05:13:02 -0800

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 <[email protected]>

  
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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