mongodb indexing subarray values -

if have in mongodb array structure this:

"field": [      [10,20]  //specified length per array      [25,40]      [60,90]      ...and on, various size per document ] 

how can index subarrays? tried index with: db.ensureindex({"field.$.0":1}); //and {"field.$.1":1}

but searching in documents remained slow.

i tried solution: mongodb, array of arrays index but

db.ensureindex("{field:[{"a":1}]}"); //and "{field:[{"b":1}]}"  

if naming subarray indexes , b, sintax throws "bad index key pattern" exception.

can explain more you're trying do? first schema design not good; have bunch of arrays have no way address except using array operators can slow.

it seems on track second idea syntax little off. if can insert documents instead of sub-arrays you'll find schema easier deal , can index on 2 values in each document illustrated below:

> db.test.insert({field: [{a:1, b:2}, {a:3, b:4}]}) > db.test.ensureindex({"field.a":1}) > db.test.ensureindex({"field.b":1}) > db.test.getindexes() [         {                 "v" : 1,                 "key" : {                         "_id" : 1                 },                 "ns" : "test.test",                 "name" : "_id_"         },         {                 "v" : 1,                 "key" : {                         "field.a" : 1                 },                 "ns" : "test.test",                 "name" : "field.a_1"         },         {                 "v" : 1,                 "key" : {                         "field.b" : 1                 },                 "ns" : "test.test",                 "name" : "field.b_1"         } ] > db.test.find({"field.a": 3}) { "_id" : objectid("520e2ec749177daf439a2ff6"), "field" : [ { "a" : 1, "b" : 2 }, { "a" : 3, "b" : 4 } ] } 

you can run explain see that, indeed, index being used (see cursor line)

> db.test.find({"field.a": 3}).explain() {         "cursor" : "btreecursor field.a_1",         "ismultikey" : true,         "n" : 1,         "nscannedobjects" : 1,         "nscanned" : 1,         "nscannedobjectsallplans" : 1,         "nscannedallplans" : 1,         "scanandorder" : false,         "indexonly" : false,         "nyields" : 0,         "nchunkskips" : 0,         "millis" : 0,         "indexbounds" : {                 "field.a" : [                         [                                 3,                                 3                         ]                 ]         },         "server" : "xxxxx-pc:27017" }