Question

for example I have collection foo of documents like that:

{"tag_cloud":[{"value":"games", "count":10}, {"value":"girls", "count":500}]}
{"tag_cloud":[{"value":"games", "count":5}, {"value":"stack", "count":100}]}
{"tag_cloud":[{"value":"mongodb", "count":1000}, {"value":"thread", "count":15}]}

what difference between two types of index:

1. ensureIndex({“tag_cloud”})
2. ensureIndex({“tag_cloud.value”}); ensureIndex({“tag_cloud.count”})

in context of request:

db.foo.find({"tag_cloud.value":"games"}).sort({"tag_cloud.count":-1});

Thank you!!!

Answer 1

MongoDB can only use one index at a time per query, so your suggestion of creating two indexes will not work. Instead, I would create a compound index:

> db.foo.ensureIndex({"tag_cloud.value": 1, "tag_cloud.count": -1})

This index can be used both to filter to only the particular element or elements you want to consider, and then also return in descending sorted order:

> db.foo.find({"tag_cloud.value":"games"}).sort({"tag_cloud.count":-1}).explain()
{
    "cursor" : "BtreeCursor tag_cloud.value_1_tag_cloud.count_-1",
    "isMultiKey" : true,
    "n" : NumberLong(2),
    "nscannedObjects" : NumberLong(2),
    "nscanned" : NumberLong(2),
    ...