Erroneous information on fetching strategies

[vince-from-nice]

Hello,

In the section 6.3.4 of the official documentation, which speaks about" Eager and Lazy Fetching", it's written that using lazy:false (int the flights association of Airport) can avoid the N+1 queries issue.

But IMHO that is not true, setting the association to eager doesn't solve the problem by itself.

Eager/Lazy just mean when the association mut be loaded: 'eager' means that the association must be loaded instantly as 'lazy' means it will be loaded only when it will be required. But what could really avoid the N+1 problem is the how it must be loaded, not the when.

By default the how is set to 'select' which means another SELECT SQL statement must be executed in order to fetch the associated entities. To avoid the N+1 problem the how must be changed to either 'join' or 'batch' or 'subselect' (apparently the last one is not possible in Grails).

I think the documentation should be corrected to avoid confusion altough it's not easy to explain Hibernate fetching strategies in a few lines...

In fact the used example deals with 3 entities: Airport, Flight and Location (it could be simpler with 2 entities only). If the flights association of Airport is set to eager, there are still N+1 queries to be executed in order to fetch relative locations ! The problem can be avoived by setting join:'fetch' on the location association of Flight.

I'm also confused with the next section about "Batch fetching". The batch mode is usefull when you load mulitple parent instances because associated entities can be fetched in one shoot for many parents (SELECT ... IN (ParentIds)). But If you have only one parent entity, which is the case of the current example, it doesn't make sense for me.

Thanks, Vincent.