Module distance 

ST_Distance and ST_DWithin — minimum distance between geometries.

Point-to-point uses haversine (great-circle distance in meters). All other combinations use planar geometry to find the nearest segment pair, then convert the coordinate-space result to meters using equirectangular approximation.

ST_DWithin uses bbox expansion as a fast pre-filter: expand query geometry’s bbox by the distance threshold, check intersection first.

Functions

st_distance

ST_Distance(a, b) — minimum distance in meters between two geometries.

st_dwithin

ST_DWithin(a, b, distance_meters) — are A and B within the given distance?