Sort on $geoWithin geospatial query in MongoDB - mongodb

Question

I'm trying to retrieve a bunch of Polygons stored inside my db, and sort them by radius. So I wrote a query with a simple $geoWithin.
So, without sorting the code looks like this:
db.areas.find(
{
"geometry" : {
"$geoWithin" : {
"$geometry" : {
"type" : "Polygon",
"coordinates" : [ [ /** omissis: array of points **/ ] ]
}
}
}
}).limit(10).explain();
And the explain result is the following:
{
"cursor" : "S2Cursor",
"isMultiKey" : true,
"n" : 10,
"nscannedObjects" : 10,
"nscanned" : 367,
"nscannedObjectsAllPlans" : 10,
"nscannedAllPlans" : 367,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 2,
"indexBounds" : {
},
"nscanned" : 367,
"matchTested" : NumberLong(10),
"geoTested" : NumberLong(10),
"cellsInCover" : NumberLong(27),
"server" : "*omissis*"
}
(Even if it's fast, it shows as cursor S2Cursor, letting me understand that my compound index has not been used. However, it's fast)
So, whenever I try to add a sort command, simply with .sort({ radius: -1 }), the query becomes extremely slow:
{
"cursor" : "S2Cursor",
"isMultiKey" : true,
"n" : 10,
"nscannedObjects" : 58429,
"nscanned" : 705337,
"nscannedObjectsAllPlans" : 58429,
"nscannedAllPlans" : 705337,
"scanAndOrder" : true,
"indexOnly" : false,
"nYields" : 3,
"nChunkSkips" : 0,
"millis" : 3186,
"indexBounds" : {
},
"nscanned" : 705337,
"matchTested" : NumberLong(58432),
"geoTested" : NumberLong(58432),
"cellsInCover" : NumberLong(27),
"server" : "*omissis*"
}
with MongoDB scanning all the documents. Obviously I tried to add a compound index, like { radius: -1, geometry : '2dsphere' } or { geometry : '2dsphere' , radius: -1 }, but nothing helped. Still very slow.
I would know if I'm using in the wrong way the compound index, if the S2Cursor tells me something I should change in my indexing strategy, overall, what I am doing wrong.
(PS: I'm using MongoDB 2.4.5+, so the problem is NOT caused by second field ascending in compound index when using 2dsphere index as reported here https://jira.mongodb.org/browse/SERVER-9647)

Answer 1

First of all, s2Cursor means that the query uses a geographic index.
There can be multiple reasons why the sort operation is slow, sort operation require memory, maybe your server has very little memory, you should consider executing sort operations in code, not at the server side.