问题
I am having a collection of documents like following:
{
"_id" : ObjectId("5bc15f23d672e9086ca4fbac"),
"Location" : {
"GeoJson" : {
"type" : "Point",
"coordinates" : [14.4199254356, 50.0700249768]
}
}
And I have Index created as following
{ Location.GeoJson2dsphere : "2dsphere" }
And now to the problem when I use the $polygon
for searching, i've got the results, but query is not using index, so it's slow. Here is the query
.find({"Location.GeoJson" : {
"$geoWithin" : {
"$polygon" : [
[14.4182910543168, 50.0891393887804],
[14.4491901021683, 50.0891393887804],
[14.4491901021683, 50.0671069088523],
[14.4182910543168, 50.0671069088523]
]
}
}
})
But when I use $Geometry instead, it is using index.
.find({"Location.GeoJson" : {
"$geoWithin" :
{"$geometry" :
{"type" : "Polygon",
"coordinates" : [[
[14.4182910543168, 50.0891393887804],
[14.4491901021683, 50.0891393887804],
[14.4491901021683, 50.0671069088523],
[14.4182910543168, 50.0671069088523],
[14.4182910543168, 50.0891393887804]
]]
}}
}})
Is there any reason why the first query isn't using the index? Mongo manual is not saying anything about this. Can you point me, what to do to use $polygon for searching index, or do I need to rewrite all the queries in my app, to use $geometry. I am using C# driver where the syntax looks like this:
Builders<Offer>.Filter.GeoWithinPolygon(a => a.Location.GeoJson, polygon);
However, this is producingf the first query, which is not using index.
回答1:
I don't work for mongo, so won't hazard a guess as to why (beyond legacy vs new data models) being confusing for newcomers, but here is how mongo differentiates between an infinite-flat-2d ($polygon query) index and a spherical index ($geometry query)
passing a $polygon targets a legacy 2d index, where or a $geometry targets the 2d-spherical index. Mongo splits out these queries to two different object types
here: https://github.com/mongodb/mongo/blob/master/src/mongo/db/geo/geoparser.cpp#L785-L790
Mongo does say that the 2dsphere index is only accessable though a $geometry query (not the legacy $polygon query)
here: https://docs.mongodb.com/manual/tutorial/query-a-2dsphere-index/
As you noted: Mongo is super helpful in that it can still return data, even if you've mismatched your queries.
Why is $polygon even a thing anymore?
Mongo continues to support non-spherical 2d indexes for those that want a 2d index on an infinite flat surface. Perhaps for video games or for any use where an infinite flat surface is a good enough approximation to avoid floating point math (and associated rounding questions)
described here:https://docs.mongodb.com/manual/tutorial/query-a-2d-index/
回答2:
Ok, and I just add the aswer in C#.
The new query is:
GeoJsonPolygon<GeoJson2DCoordinates> polygon;
var filter = Builders<Offer>.Filter.GeoWithin(a => a.Location.GeoJson, polygon);
And my helper class for rectangle.ToGeoJsonPolygon():
public class Rectangle
{
public double StartX { get; set; }
public double StartY { get; set; }
public double EndX { get; set; }
public double EndY { get; set; }
public double[,] ToCoordinates()
{
var retval = new double[4, 2];
retval[0, 0] = this.StartX;
retval[0, 1] = this.StartY;
retval[1, 0] = this.EndX;
retval[1, 1] = this.StartY;
retval[2, 0] = this.EndX;
retval[2, 1] = this.EndY;
retval[3, 0] = this.StartX;
retval[3, 1] = this.EndY;
return retval;
}
public GeoJsonPolygon<GeoJson2DCoordinates> ToGeoJsonPolygon()
{
return GeoJson.Polygon(
GeoJson.Position(StartX, StartY),
GeoJson.Position(EndX, StartY),
GeoJson.Position(EndX, EndY),
GeoJson.Position(StartX, EndY),
GeoJson.Position(StartX, StartY));
}
}
来源:https://stackoverflow.com/questions/53049304/geowithinpolygon-doesnt-use-2dsphere-index