h3-pg PostgreSQL Extension

Extension Architecture

Two extensions:

• CREATE EXTENSION h3 — Core H3 bindings (indexing, traversal, hierarchy, inspection, <-> operator). No PostGIS dependency.
• CREATE EXTENSION h3_postgis — PostGIS integration (geometry/geography casts, @ operator, polygon fill, raster helpers). Requires postgis and optionally postgis_raster.

Postgres-Specific H3 Features

These exist only in h3-pg, not other H3 language bindings.

Direct casts (h3_postgis):

• h3index::geometry → cell centroid as POINT, SRID 4326
• h3index::geography → cell centroid as POINT, SRID 4326

Operators:

• a <-> b — grid distance in cells between two h3index values (h3)
• geom @ resolution — index geometry/geography at resolution (h3_postgis)

Operator classes for h3index: BTREE, HASH, BRIN, SP-GIST (h3)

Boundary helpers (h3_postgis):

• h3_cell_to_boundary_geometry(h3index) → polygon with SRID 4326, splits at antimeridian
• h3_cell_to_boundary_geography(h3index) → geography polygon, splits at antimeridian

Polygon fill (h3_postgis):

• h3_polygon_to_cells(geometry, resolution) → SETOF h3index
• h3_polygon_to_cells(geography, resolution) → SETOF h3index

Raster integration (h3_postgis, requires postgis_raster):

• h3_raster_summary(raster, resolution, [band]) — per-cell stats (count, sum, mean, stddev, min, max)
• h3_raster_summary_clip(...) — clips raster by cell boundaries
• h3_raster_summary_centroids(...) — uses cell centroids, avoids pentagon edge cases
• h3_raster_summary_subpixel(...) — for cells smaller than pixels
• h3_raster_class_summary*(...) — discrete class counts per cell

Tile helper (h3_postgis):

• h3_get_resolution_from_tile_zoom(z, [max_h3_resolution=15], [min_h3_resolution], [hex_edge_pixels=44], [tile_size=512]) — returns optimal H3 resolution for XYZ tile zoom level z, targeting hexagons approximately hex_edge_pixels wide on screen

Common Pattern Fixes

Antimeridian gotcha: Cells crossing 180° are split into valid polygons. ST_Centroid of a split polygon may fall outside the cell — use h3::geometry for centroids instead.

Critical v3 → v4 Renames

Unit parameters (v4): Area and length functions now take a unit text parameter:

• h3_get_hexagon_area_avg(resolution, [unit = 'km^2']) — use 'km^2' or 'm^2'
• h3_get_hexagon_edge_length_avg(resolution, [unit = 'km']) — use 'km' or 'm'
• h3_cell_area(cell, [unit = 'km^2']) — exact area for specific cell
• h3_edge_length(edge, [unit = 'km']) — exact length for specific edge

Spelling change (v4.2.3+): Prefer h3_latlng_to_cell over h3_lat_lng_to_cell (underscore version deprecated). Same for h3_cell_to_latlng, h3_vertex_to_latlng.

Documentation

• API reference: https://github.com/postgis/h3-pg/blob/main/docs/api.md
• v3→v4 migration: https://h3geo.org/docs/library/migration-3.x/functions