2d Index Internals
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This document explains the internals of 2d indexes. This material is not necessary for normal operations or application development, but may be useful for troubleshooting and for further understanding.
Geohash Values
When you create a geospatial index on a field that contains legacy coordinate pairs, MongoDB computes geohash values for the coordinate pairs within the specified location range, then indexes the geohash values.
To calculate a geohash value, MongoDB recursively divides a two-dimensional map into quadrants. Then, it assigns each quadrant a two-bit value. For example, a two-bit representation of four quadrants would be:
01 11 00 10
These two-bit values (00, 01, 10, and 11) represent each
of the quadrants and all points within each quadrant. Each quadrant has
a corresponding geohash value:
Quadrant | Geohash |
|---|---|
Bottom-left | 00 |
Top-left | 01 |
Bottom-right | 10 |
Top-right | 11 |
To provide additional precision, MongoDB can divide each quadrant into
sub-quadrants. Each sub-quadrant has the geohash value of the containing
quadrant concatenated with the value of the sub-quadrant. For example,
the geohash for the top-right quadrant is 11, and the geohash for
the sub-quadrants would be (clockwise from the top left):
1101111111101100
Multi-Location Documents for 2d Indexes
While 2d indexes do not support more than one location field in a
document, you can use a multi-key index to
index multiple coordinate pairs in a single document. For example, in
the following document, the locs field holds an array of coordinate
pairs:
db.places.insertOne( { locs : [ [ 55.5 , 42.3 ], [ -74 , 44.74 ], { long : 55.5 , lat : 42.3 } ] } )
The values in the locs array may be either:
Arrays, as in
[ 55.5, 42.3 ].Embedded documents, as in
{ long : 55.5 , lat : 42.3 }.
To index all of the coordinate pairs in the locs array, create a 2d
index on the locs field:
db.places.createIndex( { "locs": "2d" } )
Embedded Multi-Location Documents
You can store location data as a field inside of an embedded document. For example, you can have an array of embedded documents where each embedded document has a field that contains location data.
In the following document, the addresses field is an array of
embedded documents. The embedded documents contain a loc field,
which is a coordinate pair:
db.records.insertOne( { name : "John Smith", addresses : [ { context : "home" , loc : [ 55.5, 42.3 ] }, { context : "work", loc : [ -74 , 44.74 ] } ] } )
To index all of the loc values in the addresses array, create a
2d index on the addresses.loc field:
db.records.createIndex( { "addresses.loc": "2d" } )