spring data JPA(Hibernate) Projections

Question

I am using Spring Boot with Spring Data JPA. In many places in my query, I am using Projections since I don't want all columns to be fetched from DB.

Projections are interface based like :

public interface VeryBasicProjection {
    String getTitle();
    String getUrl();
}

So my question is whether it actually fetches just two columns or it fetches all columns and then just set these two columns in projections?

Answer 1

Spring Data尝试选择那些实际使用的部分是最好的。

在您的情况下，它实际上应该只使用自定义SQL语句选择这两个值。

如果您的界面包含一个集合，这不起作用（没有相当复杂的查询逻辑），因此Spring Data会依赖于选择完整实体并提取投影所需的值。或者至少它会once this issue is merged。

您始终可以通过检查日志中生成的SQL来验证此行为。