31 replies

st_distance vs <-> in ash_geo for nearest neighbor search/filter (knn)

I'm just digging into

ash_geo

ash_geo

and attempting to implement a knn filter, as described here:

https://postgis.net/workshops/postgis-intro/knn.html

Unlike a distance search, the “nearest neighbour” search doesn’t include any measurement restricting how far away candidate geometries might be, features of any distance away will be accepted, as long as they are the nearest.

PostgreSQL solves the nearest neighbor problem by introducing an “order by distance” (<->) operator that induces the database to use an index to speed up a sorted return set. With an “order by distance” operator in place, a nearest neighbor query can return the “N nearest features” just by adding an ordering and limiting the result set to N entries.

I see the

st_distance

st_distance

function, in both

ash_geo

ash_geo

and

geo_postgis

geo_postgis

, but don't yet grok how to use it like the

<->

<->

operator, to get an unbounded, sorted result set.

I initially dove in with st_distance but then realized I was looking at something ~like:

  filter expr(^st_distance(^arg(:search_point), location) < some_integer_distance?) # not quite what we want

  filter expr(^st_distance(^arg(:search_point), location) < some_integer_distance?) # not quite what we want

The application idea is a user types in an address, I geocode that and get

{long, lat}

{long, lat}

, and run a knn against that to see the nearest points of interest.

Is there a

nearest

nearest

function I'm overlooking or some other way to filter and order geometries? Manually run raw SQL?

Thanks!

(Sorry if this is the wrong place, I didn't see an

ash_geo

ash_geo

tag)

Ash Framework•3y ago•

31 replies

axdc

st_distance vs <-> in ash_geo for nearest neighbor search/filter (knn)

I'm just digging into

ash_geo

ash_geo

and attempting to implement a knn filter, as described here:

https://postgis.net/workshops/postgis-intro/knn.html

Unlike a distance search, the “nearest neighbour” search doesn’t include any measurement restricting how far away candidate geometries might be, features of any distance away will be accepted, as long as they are the nearest.

PostgreSQL solves the nearest neighbor problem by introducing an “order by distance” (<->) operator that induces the database to use an index to speed up a sorted return set. With an “order by distance” operator in place, a nearest neighbor query can return the “N nearest features” just by adding an ordering and limiting the result set to N entries.

I see the

st_distance

st_distance

function, in both

ash_geo

ash_geo

and

geo_postgis

geo_postgis

, but don't yet grok how to use it like the

<->

<->

operator, to get an unbounded, sorted result set.

I initially dove in with st_distance but then realized I was looking at something ~like:

  filter expr(^st_distance(^arg(:search_point), location) < some_integer_distance?) # not quite what we want

  filter expr(^st_distance(^arg(:search_point), location) < some_integer_distance?) # not quite what we want

The application idea is a user types in an address, I geocode that and get

{long, lat}

{long, lat}

, and run a knn against that to see the nearest points of interest.

Is there a

nearest

nearest

function I'm overlooking or some other way to filter and order geometries? Manually run raw SQL?

Thanks!

(Sorry if this is the wrong place, I didn't see an

ash_geo

ash_geo

tag)

st_distance vs <-> in ash_geo for nearest neighbor search/filter (knn)

st_distance vs <-> in ash_geo for nearest neighbor search/filter (knn)

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