Question

I’m using MongoDB in my project for statistical and data analysis things. My goal is design data to have best performance and scaleability.

Let’s assume I have several shops and a list of unique products per shop. And I need to query some data about the products, calculate some basic statistic (only by curtain shop).

Which way is better from the performance point of view: to have a Shop document and a list of products inside and then make querying only per this document.

Or better will be having separate collection with all products per all shops in it and then build queries for that collection?

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Maybe the question itself: does mongodb could query through the body of one document with such efficient manner like through many documents.

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UPD 1:
For now let’s assuming that products itself is quite small (Id, Price, Name, Count) and the amount of it is limited. (So I know for sure that it won’t be more than 1000 products per shop)

UPD2
Also lets assuming that I don’t want to read that database for the view purposes, just for statistics. (How much sold, which is most interesting, what groups and so on)

Answer 1

As with all these question one of the main deciding factors is data size and growth.

Will your data per shop exceed 16 meg? Judging by how many items a shop can have and how much data can be attributed to just a single item I would yes, very quickly.

What I mean is imagine how many fields you have for a product:

• Product id
• Description
• Price
• Options
• Currency
• blurb
• SKU
• Barcode (or whatever)

Some of these fields will be quite big, For example, the description of the product could be massive.

However if on the off chance this is a very simple application and you are looking at a product that can be be fully contained in a single data row and shops which will never have more than 5-8,000 items then you could do better with subdocuments of the sort:

{
    _id: ObjectId(),
    shop_name: 'toys r us',
    items: [
        { p_id: ObjectId(), price: '1000000', currency: 'GBP', description: 'fkf' }
    ]
}

Subdocuments do not come without their price though. Imagine you have a document that only has one subdocument, in 10 days has 100 and in 20, 1000.

The fragmentation caused by the consistently growing documents could be quite significant. This lowers your performance for one. Not only will your performance become a problem but also fixing fragmentation is not a nice job and then later solving it in the applications logic is even harder.

To understand more about how MongoDB actually works inside you can view this presentation: http://www.10gen.com/presentations/storage-engine-internals

As for querying on a subdocument, it does require a little extra work on MongoDBs end but it is still quite cheap (cheaper than multiple round trips) providing you set it up right.

Personally based on the information I have given above I would go for two collections but I don’t know the true extent of your scenario…

Edit

UPD 1: For now let’s assuming that products itself is quite small (Id, Price, Name, Count) and the amount of it is limited. (So I know for sure that it won’t be more than 1000 products per shop)

Okay so your documents are small, probably a couple of bytes each. In this case you might be able to use subdocuments here with power of 2 sizes allocation to remedy some of that fragmentation: http://docs.mongodb.org/manual/reference/command/collMod/#usePowerOf2Sizes

This could create a performant operation, still 1 to 1000 subdocuments can cause fragmentation however those fragments should be filled by smaller “new” shop documents when they come into existence.

UPD2 Also lets assuming that I don’t want to read that database for the view purposes, just for statistics. (How much solds, which is most interesting, what groups and so on)

So per shop, using subdocuments, you could easily get the totals of how much sold per shop like:

db.shops.aggregate([
    // Match shop id 1
    {$match: {_id: 1}},

    // unwind the products for that shop
    {$unwind: '$products'},

    // Group back up by shop id and total amount sold
    {$group: {_id: '$_id', total_sold: {$sum: '$products.sold'}}}
])

Using the new aggregation framework (since version 2.1): http://docs.mongodb.org/manual/applications/aggregation/

So subdocuments can be just as easy as two separate collections to query on as well.