使用DTO对象休眠N + 1 SELECT

Question

I have a problem with hibernate where it is doing N+1 SELECTS when using a DTO object.

For example, this JPQL query:

SELECT com.acme.MyDto(t) FROM Thing t

Where the constructor of MyDto is something like:

public MyDto(Thing t) {
   ...
}

Which results in a query of something like:

SELECT t.id FROM thing t WHERE condition

followed by all the single queries for each row, ie:

SELECT t.id, t.column1, t.column2 FROM thing t WHERE t.id = 1
SELECT t.id, t.column1, t.column2 FROM thing t WHERE t.id = 2
SELECT t.id, t.column1, t.column2 FROM thing t WHERE t.id = 3
...

However if the constructor doesn't accept the Entity, but instead each individual column, then hibernate behaves as you would expect, ie:

public MyDto(Integer id, String column1, String column2) {
   ...
}

Then the generated SQL looks like this:

SELECT t.id, t.column1, t.column2 FROM thing t WHERE condition

Aside from creating DTO constructors that takes every column, is there a way to coax hibernate to just select all the columns at once from the beginning?

The table that we're working with has 100+ columns spread across embeddables, so it's pretty annoying to maintain a huge constructor. The table is hugely normalised and has no joins.

Answer 1

Read your question wrong the first time... I don't recall using DTOs if they just take the whole entity and not just some specific columns, so I'm not sure why Hibernate behaves like that when you use a whole entity as a parameter in DTO constructor. Anyway, you could work around it by just gettin the actual Things via a query and then construct the DTOs in a loop, something along the lines of:

public List<ThingDTO> getThingDTOs( ... )
{
    Query query = em().createQuery("FROM Thing t WHERE ...");
    query.setParameter( ... );

    List<Thing> things = query.getResultList();

    List<ThingDTO> thingDTOs = new ArrayList(things.size());
    for(Thing t : things)
    {
        thingDTOs.add(new ThingDTO(t));
    }

    return thingDTOs
}

It ain't pretty, but this way Hibernate should fetch all the needed rows in one go

Answer 2

As you have probably noticed already, the constructor expression approach has quite some downsides. If you need nested associations it going to get worse. The main problem with working with entity objects here is that you might still run into N + 1 queries problems. I wrote a blog post on that topic a while ago that justifies why I created Blaze-Persistence Entity Views , a library that allows to map DTOs as interfaces.