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<a class="user-hover" rel="steve" id="email_steve" href="https://hibernate.onjira.com/secure/ViewProfile.jspa?name=steve" style="color:#6c797f;">Steve Ebersole</a>
commented on <img src="https://hibernate.onjira.com/images/icons/newfeature.gif" height="16" width="16" border="0" align="absmiddle" alt="New Feature"> <a style='color:#6c797f;text-decoration:none;' href='https://hibernate.onjira.com/browse/HHH-5187'>HHH-5187</a>
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<a style='color:#6c797f;text-decoration:none;' href='https://hibernate.onjira.com/browse/HHH-5187'><strong>Allow some control over whether or not an association is loaded via a join on the merge path</strong></a>
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<div class="comment-block" style="background-color:#edf5ff;border:1px solid #dddddd;color:#000000;padding:12px;"><p>The concern over what Hibernate does here today is essentially a question of context. Hibernate creates and caches a load-for-merge SQL string "up front" when the <tt>SessionFactory</tt>/<tt>EntityManagerFactory</tt> is being built. Again, there are real performance reasons we do it this way; but here people are specifically asking to circumvent those performance optimizations. So first, I wanted to (re)explain there optimization here.</p>
<p>Consider a simple domain model of a <tt>Customer</tt>, <tt>Order</tt> and <tt>LineItem</tt>. And lets assume that merge cascading is enabled on <tt>Customer.orders</tt> and <tt>Order.lineItems</tt>. At start up, Hibernate will create and cache a SQL SELECT statement for loading a <tt>Customer</tt> on merge in the detached case; that SQL SELECT will "join fetch" <tt>Customer.orders</tt> and <tt>Order.lineItems</tt> because of the merge cascading. The performance gain here is 2-fold, though mostly related to avoiding the sending of multiple commands to the database. </p>
<p>However, lets consider if we really honored the idea of not loading uninitialized associations on merge. So first and foremost, this would not allow us to use the pre-cached SQL SELECT because now we are performing a more contextual execution plan. So when we receive the <tt>merge(customer)</tt> call, we now have to actually walk its association tree to determine:</p>
<ol>
<li>whether it is ok to use the pre-cached SQL SELECT based on the initialized/uninitialized state of the merge graph. So here we need to check <tt>customer.getOrders()</tt>, and assuming that is initialized, for each order we need to check <tt>order.getLineItems()</tt>.</li>
<li>in the case of any of those check finding non-initialized state we cannot use the pre-cached SQL SELECT; so we have to do "something else" which could be either:
<ol>
<li>build a modified "join fetch" SQL on the fly</li>
<li>don't try to "join fetch" at all and just use n+1 selects to load the merge graph</li>
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</li>
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<p>And even in the case of walking the association tree and determining that it is in fact ok to use the pre-cached SQL SELECT, we have still incurred the overhead of walking the tree to decide that. I almost think that, for the users wanting this, there is actually not any benefit to having the load-for-merge SQL include join fetches at all. To the point where, maybe there is just a <tt>SessionFactory</tt>-wide switch that says whether to build join fetches into the pre-cached load-for-merge SQL statements.</p>
<p>My concern with an approach like <tt>@SkipJoinLoadOn</tt> is that at the end of the day we still have a non-contextual strategy. The to-be-cached SQL SELECT would just <b>never</b> include the annotated association. Another similar option would be to have a more contextual targetted annotation (maybe <tt>@SkipJoinLoadIfUninitialized</tt>). </p></div>
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