Question

So if I have a query that does the following (in pseudo code)

find(a nearby x, b > y).sort(c)

where a is a geo column, b is type of long, and c is also a type of long

Would the compound index on (a:2d, b:1, c:1) work and suggested?

Answer 1

Geospatial queries have their own index category (as you mention), and the geohashing greatly improves the index performance of the first key lookup–it’s better than a range if you can set it up right. In any case, I think your strategy will work: the key will be setting $maxDistance to something fairly small.

I inserted 10 million random geo records to match your description, like so:

{ "_id" : ObjectId("4f28e1cffc90631d239f8b5a"), "a" : [ 46, 47 ], "b" : ISODate("2012-02-01T06:53:25.543Z"), "c" : 19 }
{ "_id" : ObjectId("4f28e1bdfc90631d239c4272"), "a" : [ 54, 48 ], "b" : ISODate("2012-02-01T06:53:32.699Z"), "c" : 20 }
{ "_id" : ObjectId("4f28e206fc90631d23aac59d"), "a" : [ 46, 52 ], "b" : ISODate("2012-02-01T06:55:14.103Z"), "c" : 22 }
{ "_id" : ObjectId("4f28e1a7fc90631d23995700"), "a" : [ 54, 52 ], "b" : ISODate("2012-02-01T06:52:33.312Z"), "c" : 27 }
{ "_id" : ObjectId("4f28e1d7fc90631d23a0e9e7"), "a" : [ 52, 46 ], "b" : ISODate("2012-02-01T06:53:11.315Z"), "c" : 31 }

With the maxDistance at something below 10 the performance is really quite good.

db.test13.find({a:{$near:[50,50], $maxDistance:4}, b:{$gt:d}}).sort({c:1}).explain();
{
"cursor" : "GeoSearchCursor",
"nscanned" : 100,
"nscannedObjects" : 100,
"n" : 100,
"scanAndOrder" : true,
"millis" : 25,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {

}
}

If you leave out maxDistance it starts to suffer quite a bit. Some of the queries took up to 60 seconds to run. The secondary range parameter doesn’t seem to help much, even if the range is fairly narrow–it seems to be all about the maxDistance.

I recommend you play around with it to get a feel for how the geospatial index performs. Here is my test insert loop. You can try limiting the bits as well for less resolution

function getRandomTime() {
   return new Date(new Date() - Math.floor(Math.random()*1000000));
}

function getRandomGeo() {
   return [Math.floor(Math.random()*360-180),Math.floor(Math.random()*360-180)];
}

function initialInsert() {
   for(var i = 0; i < 10000000; i++) {
      db.test13.save({
         a:getRandomGeo(),
         b:getRandomTime(),
         c:Math.floor(Math.random()*1000)
      });
   }
}