mongodb-indexes

问题 According to Mongo Documentation, The { item : null } query matches documents that either contain the item field whose value is null or that do not contain the item field. I can't find documentation for this, but as far as I can tell, both cases (value is null or field is missing) are stored in the index as null . So if I do db.orders.createIndex({item: 1}) and then db.orders.find({item: null}) , I would expect an IXSCAN to find all documents that either contain the item field whose value is

问题 I've tried to search through Mongo documentation, but can't really find any details on whether queries on unique indexes will be faster than queries on non-unique indexes (given the same data) So I understand that a unique index will have high selectivity and good performance. But, given two fields whose concatenation is unique, would a non-unique compound index perform slower than a unique compound index? I am assuming that unique indexes can slow down inserts as the uniqueness must be

问题 Given collection foo with the following index: db.foo.createIndex( { a : 1, b : 1, c : 1 } ) We need to select which of the following queries will use the index? db.foo.find( { c : 1 } ).sort( { a : -1, b : 1 } ) db.foo.find( { b : 3, c : 4 } ) db.foo.find( { a : 3 } ) db.foo.find( { c : 1 } ).sort( { a : 1, b : 1 } ) I'm surprised that 3 , 4 are correct options and 1 , 2 are not. Why is the following queries will use the index? db.foo.find( { a : 3 } ) This seems to be able to use the index

问题 I am trying to reproduce the first example of index intersection instruction (http://docs.mongodb.org/manual/core/index-intersection/) but facing a problem: mongo doesn't uses both indexes My steps: Download mongo (3.0.3) and install it Run mongod: mongod.exe --dbpath d:\data (folder is empty) Run mongo: mongo.exe Add indexes: db.orders.ensureIndex({ qty: 1 }) db.orders.ensureIndex({ item: 1 }) db.orders.getIndexes() [{ "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "test.orders" },

问题 Consider the below Mongo index strategy and the query, Index: db.collec.ensureIndex({a:1,b:1,c:1}); Query: db.collec.find({"a":"valueA"},{"_id":0,"a":1,"c":1}).sort({"c":-1}).limit(150) The explain on the above query returns: /* 0 */ { "cursor" : "BtreeCursor a_1_b_1_c_1", "isMultiKey" : false, "n" : 150, "nscannedObjects" : 178, "nscanned" : 178, "nscannedObjectsAllPlans" : 279, "nscannedAllPlans" : 279, "scanAndOrder" : true, "indexOnly" : true, "nYields" : 0, "nChunkSkips" : 0, "millis" :

问题 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

问题 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

问题 I was creating my first app using MongoDB. Created index for a field, and tried a find query with $regex param, launched in a shell > db.foo.find({A:{$regex:'BLABLA!25500[0-9]'}}).explain() { "cursor" : "BtreeCursor A_1 multi", "nscanned" : 500001, "nscannedObjects" : 10, "n" : 10, "millis" : 956, "nYields" : 0, "nChunkSkips" : 0, "isMultiKey" : false, "indexOnly" : false, "indexBounds" : { "A" : [ [ "", { } ], [ /BLABLA!25500[0-9]/, /BLABLA!25500[0-9]/ ] ] } } It's very strange, because when

问题 There will be about 3 billion docs in the collection across shards. Assuming I would be using queries that is not fulfilled entirely by the index(es). And I am not using the keys for sorting. Does it make sense to have indexes as follows (which match the queries): {"sid":1, "cid":1, "mid":1} {"cid":1, "hid":1, "mid":1} {"mid":1, "hid":1, "sid":1} {"hid":1, "sid":1, "cid":1, "mid":1} {"mid":1, "cid":1} {"mid":1, "sid":1} Or: {sid:1} {cid:1} {hid:1} {mid:1} 回答1: This is a valid question. You

问题 I have a document with one field description like this: { "_id": "item0", "description": { "parlist": [ { "listitem": { "text": { "child": "page rous lady", "keyword": "officer e" } } }, { "listitem": { "text": "shepherd noble " } } ] } } How to create text index on description and search for specific word? I don't know how depth can description go and how many children will description have. I tried with index creation like this: db.collection.ensureIndex({description:"text"}) and then for