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https://issues.apache.org/jira/browse/CAY-1681?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13236564#comment-13236564
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Andrei Veprev edited comment on CAY-1681 at 3/23/12 1:03 PM:
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Attaching a patch with DISJOIN_BY_ID prefetch implementation.
After some discussion with Andrus, the decision was to do all prefetch queries
for that type of prefetch on the object conversion stage of query execution
(conversion from data rows to objects). So here it is, most of the staff are
done inside HierarchicalObjectResolver.
Flatten relationships are not implemented yet, as well as support for phantoms
in a middle of prefetch tree hierarchy (marked as TODO in the patch).
was (Author: aveprev):
Attaching a patch with DISJOIN_BY_ID prefetch implementation.
After some discussion with Andrus, the decision was to do all prefetch queries
for that type of prefetch on the object conversion stage of query execution
(conversion from data rows to objects). So here it is, most of the staff done
inside HierarchicalObjectResolver.
Flatten relationships are not implemented yet, as well as support for phantoms
in a middle of prefetch tree hierarchy (marked as TODO in the patch).
> Third prefetch kind - DISJOINT_BY_ID
> ------------------------------------
>
> Key: CAY-1681
> URL: https://issues.apache.org/jira/browse/CAY-1681
> Project: Cayenne
> Issue Type: Task
> Components: Core Library
> Reporter: Andrus Adamchik
> Assignee: Andrus Adamchik
> Attachments: CAY-1681.patch
>
>
> (here is a mailing list thread discussing the issue:
> http://markmail.org/message/zzyd26ucfwhnacfe )
> I keep encountering a common scenario where neither JOINT or DISJOINT
> prefetch strategies are adequate - queries with fetch limit. It is very
> common in the application to display X most recent entries from a table with
> millions of rows, and then drill down to the object details. E.g. assume 2
> entities - "Order" and "LineItem", with orders having multiple line items. We
> want 10 most recent orders, with line items prefetched, so you'd so something
> like this:
> SelectQuery q = new SelectQuery(Order.class);
> q.addPrefetch("lineItems");
> q.setFetchLimit(10);
> "Disjoint" prefetch in this situation would fetch 10 orders and ALL LineItems
> in DB.
> "Joint" prefetch will fetch anywhere between 1 and 10 orders, depending on
> how many line items the first 10 orders have, i.e. fetch limit is applied to
> to-many join, not to the query root. And this is certainly not what we want.
> Now Cayenne already has something that can solve the problem:
> q.setPageSize(10); // same as fetch limit
> Paginated query is the most optimal way to prefetch here. Whenever a result
> list is accessed, Cayenne would execute 2 IN () queries - one for the Orders,
> another one - for the LineItems. Both queries are matching on a set of Order
> PKs and are pretty efficient, and only return the objects that we care about.
> The problem with this solution is that it is counterintuitive to the user
> (why should I set "pageSize" to make my prefetches work) and adds one extra
> query (the IN query resolving the root object list). Would be cool to turn it
> into a separate type of prefetch. Something like "disjoint by id"?
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