Nearest Feature (KNN /

<->

)

<->

When to use

• “Find the nearest X to this point”
• “Rank candidates by proximity”
• “Pick the closest feature per input row”

Core rules

• KNN

  <->

  works best with GiST indexes on geometry columns

• <->

  orders by distance in the geometry’s coordinate system (SRID units)
• For meaningful distances, use a projected SRID or

  geography

  (meters)
• Keep queries index-friendly: avoid wrapping the indexed column in transforms inside ORDER BY when possible

Index requirement

CREATE INDEX IF NOT EXISTS features_geom_gix ON features USING gist (geom);

Canonical pattern: nearest feature to a single point

-- point is EPSG:4326 here
WITH p AS (
  SELECT ST_SetSRID(ST_MakePoint($1, $2), 4326) AS geom
)
SELECT f.*
FROM features f, p
ORDER BY f.geom <-> p.geom
LIMIT 1;

Nearest N features

WITH p AS (
  SELECT ST_SetSRID(ST_MakePoint($1, $2), 4326) AS geom
)
SELECT f.*
FROM features f, p
ORDER BY f.geom <-> p.geom
LIMIT 10;

Add an exact distance (optional)

WITH p AS (
  SELECT ST_SetSRID(ST_MakePoint($1, $2), 4326) AS geom
)
SELECT
  f.*,
  ST_Distance(f.geom::geography, p.geom::geography) AS distance_m
FROM features f, p
ORDER BY f.geom <-> p.geom
LIMIT 1;

• ordering stays fast via KNN
• distance is computed in meters via geography

Recommended: constrain candidates with ST_DWithin

WITH p AS (
  SELECT ST_SetSRID(ST_MakePoint($1, $2), 4326) AS geom
)
SELECT f.*
FROM features f, p
WHERE ST_DWithin(f.geom::geography, p.geom::geography, 5000) -- 5km
ORDER BY f.geom <-> p.geom
LIMIT 1;

Different SRIDs

WITH p AS (
  SELECT ST_Transform(
    ST_SetSRID(ST_MakePoint($1, $2), 4326),
    $3  -- client_srid
  ) AS geom
)
SELECT f.*
FROM features f, p
ORDER BY f.geom <-> p.geom
LIMIT 1;

features.geom

Common mistakes

• No GiST index (query is slow)
• Using

  <->

  on EPSG:4326 and assuming the distance is meters
• Transforming

  f.geom

  inside ORDER BY (can kill index usage)
• Not bounding the search (use ST_DWithin when appropriate)

Summary

• Use

  ORDER BY geom <-> point LIMIT 1

  for nearest neighbor
• Ensure GiST index on

  geom

• Use geography or projected SRID for real-world distance values
• Add ST_DWithin to keep searches local and fast