How does mongodb index lists

Question

For example: If I had a db collection called Stores, and each store document has a list of the items they sell, and stores generally share items, then how would mongodb build an index on that? Would it build a btree index on all possible items and then on each leaf of that tree (each item) will reference the documents which contain it?

Background: I'm trying to perform queries like this using an index:

db.store.find({merchandise:{$exists:true}}) // where 'merchandise' is a list
db.store.find()[merchandise].count()

would an index on 'merchandise' help me? If not, is my only option creating a separate meta field on 'merchandise' size, and index that?

Schema:

{ _id: 123456,
  name: Macys
  merchandise: [ 248651234564, 54862101248, 12450184, 1256001456 ]
}

When you want to know if a certain query is aided by an index or not, .explain() is your friend. — Philipp
– Philipp, Commented Sep 3, 2013 at 20:45

fgakk · Accepted Answer · 2013-09-04 05:55:15Z

1

From your document sample if you build your index on merchandise it will be multikey index and that index will be on every item on the array. See Multikey Indexes section in here.

If merchandise is an array of subdocuments, indexing over merchandise will put the index on all field of subdocument in the array. With index you can make queries like db.store.find("merchandise":248651234564) and it will retrieve all document having merchandise 248651234564

For getting count of merchandise, you can get only get the size of merchandise field of one document like db.store.find()[index].merchandise.length. So creating a seperate field on merchandise size and indexing is a feasible option, if you want to run queries based on merchandise size.

Hope this helps

edited Sep 4, 2013 at 5:55

answered Sep 3, 2013 at 21:02

fgakk

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Comments

Dyin · Accepted Answer · 2013-09-04 08:03:47Z

If you index a field that contains an array, MongoDB indexes each value in the array separately, in a multikey index. When you have 4 documents inside an array, each will act as a key in the index and point to the mentioned document(s).

You can use multikey indexes to index fields within objects embedded in arrays. That means, in your array, you can index a specific field in each document. For example: stuffs.thing : 1.

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