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
