Q 4 C. Bauer & G. King (6): "association fetching" and fetching strategy

[Gian Franco]

Dear authors,

In the thread you mention association fetching, this means, I guess, that
Hibernate has to have an overview of all relations that compose
an object we would like to manipulate. How is the data retrieved,
table by table or does Hibernate optimize it's fetching?

Cheers,

Gian Franco Casula

[ August 31, 2004: Message edited by: Gian Franco Casula ]
[ August 31, 2004: Message edited by: Gian Franco Casula ]

[Gavin King]

Actually, you optimize the fetching strategy, by specifying, in mapping documents, and/or queries, whether you want an association fetched by a lazy sequential select when it is accessed, or by an outer join. In addition you can enable or disable "batch" fetching.

[Christian Bauer]

There are many ways to optimize association fetching, depending on the business case and the amount of data required:

1. Enable the second-level cache and hope that associated instances will mostly be found in the cache. There are situations when a cache can't (and shouldn't) be used, so you can tune this on a per-class and per-association basis.

2. Lazy load and Eager load associated entities. The first means that an associated entity is only loaded when access for the first time, the latter that a "prefetch" is executed. An eager fetch query tries to retrieve as much data as possible (or sensible) in a single query. There are several options how both strategies can be used as default (for graph navigation) and at runtime in HQL/Criteria queries.

3. Optimizations using batch fetching (not to be confused with JDBC batch operations) for "similar" collections and entities.

4. Some more minor optimization techniques.

[somkiat puisungnoen]

What does "Lazy load" mean ???

[somkiat puisungnoen]

When I should be use Lazy Fetching ?

When I should be use "batch" fetching ?

[Alexandru Popescu]

Lazy load=

There are many ways to optimize association fetching, depending on the business case and the amount of data required:

2. Lazy load and Eager load associated entities. The first means that an associated entity is only loaded when access for the first time, [...]