<<->> — Returns the n-D distance between the centroids of A and B bounding boxes.
double precision <<->>(
<<->> operator returns the n-D (euclidean)
distance between the centroids of the bounding boxes of two geometries.
Useful for doing nearest neighbor
approximate distance ordering.
This operand will make use of n-D GiST indexes that may be available on the geometries. It is different from other operators that use spatial indexes in that the spatial index is only used when the operator is in the ORDER BY clause.
Index only kicks in if one of the geometries is a constant (not in a subquery/cte). e.g. 'SRID=3005;POINT(1011102 450541)'::geometry instead of a.geom
Availability: 2.2.0 -- KNN only available for PostgreSQL 9.1+