MongoDB-输出距离，无需排序

Question

I am new to MongoDB and I am jumping right to the deep water of geo-spatial queries. I noticed that the geoNear command and the $geoNear aggregation operator can output the calculated distance (using distanceField ).

The command and the operator also sort the results by distance.

Is there a way to output the distance without sorting by distance? ie use some projection operator. This could obviously be done using a map with a javascript function to calculate the distance manually, but it seems there is a built-in distance calculation function that must be much more efficient. Also, it seems to be inefficient to use the $geoNear operator and afterwards (in the aggregation pipeline) resort the results... right?

Thanks

Answer 1

Due to the num and limit parameters to a $geoNear query, (which are mutually exclusive -- with num overriding limit if both are present, see docs ), the results have to be sorted by distance to determine which ones to return. So, in fact, no extra work is being done in returning the results sorted. As far as I know, it is not possible to return results unsorted by distance.

This makes particular sense in the case of sharding, which cannot currently be done on a geo key , as the query has to be sent to all shards, and the results then combined and returned according to the values of num and/or limit.

MongoDB does not use an R-tree index, as some spatial databases do, which can make distance searches somewhat more efficient, but a B-tree on a geohash, so a distance query would work by comparing the distances in the nearest quadrants, until a sufficient number are found, which is likely to be more than num or limit and then returning only that number, after sorting on distance.