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mention @ConcreteProxy and the problem it solves in Introduction doc 

Signed-off-by: Gavin King <gavin@hibernate.org>
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Gavin King 2024-07-11 13:15:41 +02:00
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documentation/src/main/asciidoc/introduction/Interacting.adoc
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@ -330,9 +330,15 @@ Now for the gotchas:
1. Hibernate will only do this for an entity which is currently associated with a persistence context. 1. Hibernate will only do this for an entity which is currently associated with a persistence context.
Once the session ends, and the persistence context is cleaned up, the proxy is no longer fetchable, and instead its methods throw the hated `LazyInitializationException`. Once the session ends, and the persistence context is cleaned up, the proxy is no longer fetchable, and instead its methods throw the hated `LazyInitializationException`.
2. A round trip to the database to fetch the state of a single entity instance is just about _the least efficient_ way to access data. 2. For a polymorphic association, Hibernate does not know the concrete type of the referenced entity when the proxy is instantiated, and so operations like `instanceof` and typecasts do not work correctly when applied to a proxy.
3. A round trip to the database to fetch the state of a single entity instance is just about _the least efficient_ way to access data.
It almost inevitably leads to the infamous _N+1 selects_ problem we'll discuss later when we talk about how to <<association-fetching,optimize association fetching>>. It almost inevitably leads to the infamous _N+1 selects_ problem we'll discuss later when we talk about how to <<association-fetching,optimize association fetching>>.
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The link:{doc-javadoc-url}org/hibernate/annotations/ConcreteProxy.html[`@ConcreteProxy`] annotation solves gotcha 2, but at the cost of performance (extra joins), and so its use is not generally recommended, except in very special circumstances.
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// .Strive to avoid triggering lazy fetching // .Strive to avoid triggering lazy fetching
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