postgis-query-patterns

name	postgis-query-patterns
description	Use when writing PostGIS or spatial SQL — distance, proximity, intersection, or geometry storage queries. Makes the agent use correct SRIDs, spatial indexes, and the right ST_ functions, so spatial SQL is both correct and fast instead of silently slow or wrong.

PostGIS Query Patterns

PostGIS rewards correct spatial SQL and quietly punishes the rest with wrong answers or full table scans. Follow the patterns.

Storage & SRID

• Store geometry with a known SRID (typmod like geometry(Point, 4326) or ST_SetSRID). Operations between different SRIDs error or mislead — ST_Transform to align.
• Choose geography for accurate distances over large/global areas (meters on a sphere); geometry in an appropriate projected CRS for fast planar math.

Make queries use the index

• Put a GiST index on every geometry column you query.
• Use index-assisted operators/functions: &&, ST_Intersects, ST_DWithin. These hit the index.
• Avoid ST_Distance(a, b) < x in a WHERE clause — it can't use the index and forces a full scan. Use ST_DWithin(a, b, x) instead.

Correctness gotchas

• && is bounding-box only — fast but approximate. Use it as a prefilter, then ST_Intersects for the exact test.
• Validate geometry before overlays (ST_IsValid / ST_MakeValid) — see validate-geometry.
• ST_DWithin distance units follow the type: meters for geography, CRS units for geometry (degrees if you left it in 4326 — usually not what you want for a distance).

Why this matters

Spatial SQL fails quietly in two directions: wrong SRID or wrong function gives a confident wrong answer, and a missing index turns a sub-second query into a minutes-long table scan that still "works" in testing and falls over in production. The patterns above keep queries both correct and fast.