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From Andrus Adamchik <and...@objectstyle.org>
Subject Third prefetch kind - DISJOINT_BY_ID
Date Mon, 30 Jan 2012 08:37:54 GMT
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"?

Andrus




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