ST_DWithin (Distance / Proximity Queries)

Use this skill when you need to find features within a given distance of another geometry:

• “points within X meters of a line”

• proximity searches around a point

• replacing expensive buffer + intersects patterns

Core rules

• ST_DWithin is faster than ST_Distance < x

• Units depend on input type:

• geometry → SRID units

• geography → meters

• Geometries must be in the same SRID

• Prefer ST_DWithin over buffering when you only need a boolean test

Canonical patterns

Geometry (projected CRS, SRID units)

Use when your data is stored in a projected SRID (feet/meters).

SELECT * FROM points p JOIN lines l ON ST_DWithin(p.geom, l.geom, 5);

Distance 5 is in the SRID’s units.

Geography (meters, easiest default)

Use when geometry is stored in EPSG:4326 and you want meters.

SELECT * FROM points p JOIN lines l ON ST_DWithin( p.geom::geography, l.geom::geography, 5 );

Distance 5 is meters.

Transform once (projected math, better for large jobs)

When using a client/project SRID:

SELECT * FROM points p JOIN lines l ON ST_DWithin( ST_Transform(p.geom, client_srid), ST_Transform(l.geom, client_srid), 5 );

Transforming both sides is acceptable here because ST_DWithin can still use indexes in many cases, but prefer transforming the smaller side if possible.

Point near line (common validation pattern)

SELECT p.* FROM points p JOIN lines l ON ST_DWithin( p.geom::geography, l.geom::geography, 5 );

Use this instead of buffering lines unless you need the actual corridor geometry.

Index requirements

Ensure GiST indexes exist:

CREATE INDEX IF NOT EXISTS points_geom_gix ON points USING gist (geom); CREATE INDEX IF NOT EXISTS lines_geom_gix ON lines USING gist (geom);

ST_DWithin can leverage spatial indexes when used correctly.

Common mistakes

• Using ST_Distance(...) < x instead of ST_DWithin

• Forgetting geometry vs geography unit differences

• Mixing SRIDs without transforming

• Buffering geometries just to test proximity

Summary

• Prefer ST_DWithin for distance tests

• Use geography for meters when in 4326

• Use projected SRIDs for heavy planar work

• Index geometry columns