n+1 select & fetch strategy

The n+1 select is for return *LIST* of objects, and the object contains collection. Then we can use 'subselect' fetch strategy to reduce the number of queries. For example, Template (1-->*) DeployedController, with default fetch strategy 'select', to return a list of template it requires 1 + n selects.

select * from template;
select * from deployedController where templateId=?
select * from deployedController where templateId=?
...

With 'subselect', it becomes only 2 queries -

select * from template;
select * from deployedController where templateId in (select id from template)

This is for lazy loading, the 2nd query is run when the 1st time the deployedController collections is accessed. For eager loading, change fetch strategy to 'join', then only one query (left outer join) is used but the resultset contains duplicate or null values.

The subselect option is currently only available for collection, not for *-->1 assocation.

Eager loading:
1. lazy="false", stands for fetch="select", follow by an immediate second select
2. fetch="join"
Lazy Loading:
1. fetch="subselect"
2. batch-size="N"