I have a collection called search2 with about 20000 documents like this:
{
"loc": {
"type": "Polygon",
"coordinates": [
[
[
43.78526674007639,
11.14739998758569
],
[
43.78526674007639,
11.183372851822439
],
[
43.79443488391605,
11.183372851822439
],
[
43.79443488391605,
11.264311796355125
],
[
43.812771171595415,
11.264311796355125
],
[
43.83110745927479,
11.264311796355125
],
[
43.83110745927479,
11.273305012414314
],
[
43.849443746954144,
11.273305012414314
],
[
43.858611890793824,
11.273305012414314
],
[
43.858611890793824,
11.264311796355125
],
[
43.8769481784732,
11.264311796355125
],
[
43.8769481784732,
11.246325364236752
],
[
43.88611632231286,
11.246325364236752
],
[
43.88611632231286,
11.237332148177565
],
[
43.895284466152546,
11.237332148177565
],
[
43.895284466152546,
11.228338932118376
],
[
43.904452609992234,
11.228338932118376
],
[
43.904452609992234,
11.165386419704065
],
[
43.895284466152546,
11.165386419704065
],
[
43.895284466152546,
11.156393203644878
],
[
43.88611632231286,
11.156393203644878
],
[
43.8769481784732,
11.156393203644878
],
[
43.858611890793824,
11.156393203644878
],
[
43.849443746954144,
11.156393203644878
],
[
43.849443746954144,
11.165386419704065
],
[
43.83110745927479,
11.165386419704065
],
[
43.83110745927479,
11.156393203644878
],
[
43.812771171595415,
11.156393203644878
],
[
43.812771171595415,
11.14739998758569
],
[
43.79443488391605,
11.14739998758569
],
[
43.78526674007639,
11.14739998758569
]
]
]
},
"docId": 1,
"docVote": 0,
"title": "title-1",
"_id": {
"$oid": "5248725d2dd5622510000001"
}
}
I define an index with this command:
db.search2.ensureIndex({"docVote": 1,"loc":"2dsphere"});
On the collection there are only this index and the default index on "_id" field.
When i execute the following query i expect "nscannedObjects" to be = 10 :
db.search2.find({
loc: {
$geoIntersects: {
$geometry: {
type: "Polygon",
coordinates: [
[
[43.7269795, 11.1540365],
[43.8329368, 11.1540365],
[43.8329368, 11.3310908],
[43.7269795, 11.3310908],
[43.7269795, 11.1540365]
]
]
}
}
}
}, {
"docVote": 1,
_id: 0
}).sort({
"docVote": 1
}).limit(10).hint({
"docVote": 1,
"loc": "2dsphere"
}).explain()
But this is the result:
{
"cursor" : "S2Cursor",
"isMultiKey" : true,
"n" : 10,
"nscannedObjects" : 44283,
"nscanned" : 648117,
"nscannedObjectsAllPlans" : 44283,
"nscannedAllPlans" : 648117,
"scanAndOrder" : true,
"indexOnly" : false,
"nYields" : 13,
"nChunkSkips" : 0,
"millis" : 12632,
"indexBounds" : {
},
"nscanned" : 648117,
"matchTested" : NumberLong(46642),
"geoTested" : NumberLong(46642),
"cellsInCover" : NumberLong(8),
"server" : "*********"
}
If i remove sort from the query i obtain this:
{
"cursor" : "S2Cursor",
"isMultiKey" : true,
"n" : 10,
"nscannedObjects" : 10,
"nscanned" : 25,
"nscannedObjectsAllPlans" : 10,
"nscannedAllPlans" : 25,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 3,
"indexBounds" : {
},
"nscanned" : 25,
"matchTested" : NumberLong(10),
"geoTested" : NumberLong(10),
"cellsInCover" : NumberLong(8),
"server" : "******"
}
So, why index is not used to sort results? From this documentation: http://docs.mongodb.org/manual/applications/geospatial-indexes/ http://docs.mongodb.org/manual/tutorial/sort-results-with-indexes/ I understand that MongoDb support "A compound index with scalar index fields (i.e. ascending or descending) as a prefix or suffix of the 2dsphere index field" and that "If the sort document is a subset of a compound index and starts from the beginning of the index, MongoDB can use the index to both retrieve and sort the query results."
What am I missing?
Thanks in advance
