Proper way to fetch some docs from DB (Mongo) - node.js

Question

Hello and thanks in advance!
MERN stack student here. I'm just getting to know the MVC design pattern aswell.
So, here's my question:
I'm trying to get some docs from a Mongo collection (working with Mongoose) and I get a limit query with my request (let's say I need only the first 5 docs from a collection of 30 docs). What is considered best practice in general? Would you use one or the other in different cases (i.e. how big is the db, as an example that comes to mind)?
Something like this:
Controller:
getProducts() {
const { limit } = req.query;
const products = await productManagerDB.getProducts();
res.status(200).json({ success: true, limitedProductsList: products.slice(0, Number(limit))});
}
Or
Like this:
Controller:
getProducts() {
const { limit } = req.query;
const products = await productManagerDB.getProducts(limit);
res.status(200).json({ success: true, limitedProductsList: products});
}
Service:
getProducts(query) {
try {
const limit = query? Number(query) : 0;
const products = await ProductsModel.find().limit(limit);
return products;
} catch (error) {
throw new Error(error.message)
}
}
Tried both ways with the same outcome. I expect second to be more efficient since it's not loading all the data that I ain't using but curious if in some cases would be better to fetch the whole collection...

Answer 1

As you have already stated the second query is far more efficient - especially when you have a large collection.
the two differences would be:
MongoDB engine will have to fetch all the documents from the disk -
the way MongoDB is designed (the internal wired-tiger storage
engine) is that it caches the frequently used data either in the
application cache or the OS cache - in any case, its quite possible
that the whole collection will not fit in the memory and therefore a
large number of disk operations will happen (very slow comparatively
even with the latest nvme disks)
A large amount of data will have to flow over the network from the
database server to the application server which is a waste of
bandwidth and will be slower.
Where you might need the full collection obviously depends on the usecase