Thomas.Ruecksti
...@10gen.com> wrote:
> Hi Tom,
> according to the documentation, "Queries cannot use indexes for the $size<http://docs.mongodb.org/manual/reference/operators/#_S_size>portion
> of a query".
> http://docs.mongodb.org/manual/reference/operator/size/
> In order to make a query on the number of documents in an array efficient,
> you could add a separate field 'numCapturedDates' to your schema and
> manually increase and decrease it when you add/remove elements to/from the
> array. An index on 'numCapturedDates' would then help you efficiently
> retrieve documents with a certain number of elements.
> Regards,
> Thomas
> On Friday, September 7, 2012 1:06:38 PM UTC+10, Tom Vo wrote:
>> Dear all
>> *I have collection Devices with data sample below.*
>> db.Devices.find()
>> {
>> "_id" : NumberLong(1),
>> "CapturedDate" : [
>> [
>> 1,
>> ISODate("2012-05-10T02:07:50.**159Z")
>> ],
>> [
>> 2,
>> ISODate("2012-05-09T12:31:50.**299Z")
>> ]
>> ],
>> "OSName" : "Windows",
>> "DeviceType" : "Mobile"
>> }
>> {
>> "_id" : NumberLong(2),
>> "CapturedDate" : [
>> [
>> 1,
>> ISODate("2012-04-10T06:59:50.**123Z")
>> ],
>> [
>> 3,
>> ISODate("2012-12-09T12:06:50.**243Z")
>> ]
>> ],
>> "OSName" : "Iphone",
>> "DeviceType" : "Mobile"
>> }
>> *I want to find size of "CapturedDate" great than 2, and this query run
>> slowly. **I try to create index on "CapturedDate" but performance not
>> improve. Please give me to your suggestion to solute this trouble.*
>> db.Devices.ensureIndex( { CapturedDate : 1 } )
>> db.Devices.find({"**CapturedDate":{$size:2}},{**
>> CapturedDate:1}).explain()
>> {
>> "cursor" : "BasicCursor",
>> "nscanned" : 1218300,
>> "nscannedObjects" : 1218300,
>> "n" : 73,
>> "millis" : 21544,
>> "nYields" : 0,
>> "nChunkSkips" : 0,
>> "isMultiKey" : false,
>> "indexOnly" : false,
>> "indexBounds" : {
>> }
>> }
>> > db.Devices.find( { $where: "this.CapturedDate.length > 1" }
>> ).hint({CapturedDate : 1}).explain()
>> {
>> "cursor" : "BtreeCursor CapturedDate_1",
>> "nscanned" : 1218376,
>> "nscannedObjects" : 1218376,
>> "n" : 73,
>> "millis" : 23276,
>> "nYields" : 0,
>> "nChunkSkips" : 0,
>> "isMultiKey" : true,
>> "indexOnly" : false,
>> "indexBounds" : {
>> "CapturedDate" : [
>> [
>> {
>> "$minElement" : 1
>> },
>> {
>> "$maxElement" : 1
>> }
>> ]
>> ]
>> }
>> *Thanks and Regards*
>> *Tom Vo*
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