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https://issues.apache.org/jira/browse/CAY-1681?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13243699#comment-13243699
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Andrus Adamchik commented on CAY-1681:
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Another interesting thing that I noticed. From 'testManyToOne':
SELECT t0.ID, t0.NAME FROM BAG t0 WHERE (t0.ID = ?) OR (t0.ID = ?) [bind:
1->ID:1, 2->ID:1]
As you see there are two conditions checking the same thing. Wonder if it is
worthwhile to collapse repeating conditions in the where clause (i.e. whether
overhead we add in Java would pay off by creating a more efficient DB query)..
Just something to ponder. Not a must-have certainly.
> 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-v2.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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