hibernate-batch-size

The main goal of this project is to provide simple solution to N+1 query problem.

preface

A naive O/R implementation would SELECT entity and then do additional SELECTs for getting the information of child entities, so in case of (ex. @OnToMany) mapped collection of child entities (size = N) causes N+1 query problem.

solution

@BatchSize - Defines size for batch loading of collections or lazy entities.

Without @BatchSize, you’d run into a N+1 query issue.

However, although @BatchSize is better than running into an N+1 query issue, most of the time, a DTO projection or a JOIN FETCH is a much better alternative since it allows you to fetch all the required data with a single query.

example

over entity
```
@Entity
@BatchSize(size=100)
class Product {
    ...
}
```
will initialize up to 100 lazy Product entity proxies at a time.
over relation
```
@OneToMany
@BatchSize(size = 5) /
Set getProducts() { ... };
```
will initialize up to 5 lazy collections of products at a time

tests

Running withBatch() tests produces sql queries:

Hibernate: select employee0_.id as id1_0_0_ from employee employee0_ where employee0_.id=?
Hibernate: select employee0_.id as id1_0_0_ from employee employee0_ where employee0_.id=?
Hibernate: select employee0_.id as id1_0_0_ from employee employee0_ where employee0_.id=?

Hibernate: select batchissue0_.batch_issues_id as batch_is3_1_1_, batchissue0_.id as id1_1_1_, batchissue0_.id as id1_1_0_ from issue batchissue0_ where batchissue0_.batch_issues_id in (?, ?, ?)

Running withoutBatch() tests produces sql queries:

Hibernate: select employee0_.id as id1_0_0_ from employee employee0_ where employee0_.id=?
Hibernate: select employee0_.id as id1_0_0_ from employee employee0_ where employee0_.id=?
Hibernate: select employee0_.id as id1_0_0_ from employee employee0_ where employee0_.id=?

Hibernate: select notbatchis0_.not_batch_issues_id as not_batc2_1_0_, notbatchis0_.id as id1_1_0_, notbatchis0_.id as id1_1_1_ from issue notbatchis0_ where notbatchis0_.not_batch_issues_id=?
Hibernate: select notbatchis0_.not_batch_issues_id as not_batc2_1_0_, notbatchis0_.id as id1_1_0_, notbatchis0_.id as id1_1_1_ from issue notbatchis0_ where notbatchis0_.not_batch_issues_id=?
Hibernate: select notbatchis0_.not_batch_issues_id as not_batc2_1_0_, notbatchis0_.id as id1_1_0_, notbatchis0_.id as id1_1_1_ from issue notbatchis0_ where notbatchis0_.not_batch_issues_id=?

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