mongodb-indexes

In mongodb there are multiple types of index . For this question I'm interested in the ascending (or descending) index which can be used for sorting and the hash index which according to the documentation is "primarily used with sharded clusters to support hashed shard keys" ( source ) ensuring "a more even distribution of data"( source ) I know that you can't create an index like: db.test.ensureIndex( { "key": "hashed", "sortOrder": 1 } ) because you get an error { "createdCollectionAutomatically" : true, "numIndexesBefore" : 1, "errmsg" : "exception: Currently only single field hashed index

I have a MongoDB collection named post with 35 million objects. The collection has two secondary indexes defined as follows. > db.post.getIndexKeys() [ { "_id" : 1 }, { "namespace" : 1, "domain" : 1, "post_id" : 1 }, { "namespace" : 1, "post_time" : 1, "tags" : 1 // this is an array field } ] I expect the following query, which simply filters by namespace and post_time , to run in a reasonable time without scanning all objects. >db.post.find({post_time: {"$gte" : ISODate("2013-04-09T00:00:00Z"), "$lt" : ISODate("2013-04-09T01:00:00Z")}, namespace: "my_namespace"}).count() 7408 However, it

I'm finding it hard to understand how exactly indexing is done on multikeys in mongodb. This is what I read about multikeys in mongodb docs on its website: 1) "Creating an index on an array element indexes results in the database indexing each element of the array" 2) "...will index all the tags on the document, and create index entries for "X", "Y" and "Z" for that document." So what exactly does it mean by index entries for that document? Does each doc remember the entries, in which case searching is gonna be a full table scan? Or is it the same b-tree index of mysql where each index entry

I'm implementing a contact database that handles quite a few fields. Most of them are predefined and can be considered bound, but there are a couple that aren't. We'll call one of these fields 'groups'. The way we currently implement it is (each document/contact has 'groups' field): 'groups' : { 152 : 'hi', 111 : 'group2' } but after some reading I've it would seem I should be doing it: 'groups' : [ { 'id' : 152, 'name' : 'hi' }, { 'id' : 111, 'name' : 'group2' } ... ] and then apply the index db.contact.ensureIndex({'groups.id':1}); My question is in regard to functionality. What are the