name	h3-pg
description	PostgreSQL bindings for H3 hexagonal grid system. Use when working with H3 cells in Postgres, including spatial indexing, geometry/geography integration, and raster analysis.

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)

Inspection and construction helpers (h3):

h3_get_index_digit(cell, resolution) — inspect the digit at a 1-based resolution step
h3_construct_cell(resolution, base_cell_number, digits[]) — rebuild a cell from explicit components
h3_is_valid_index(index) — validate any H3 index mode: cell, directed edge, or vertex

Directed edge convenience (h3):

h3_reverse_directed_edge(edge) — returns the same directed edge with origin and destination reversed

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

Pattern	DO NOT	DO
Get centroid	`ST_Centroid(h3_cell_to_boundary_geometry(h3))`	`h3::geometry`
Get boundary with SRID	`h3_cell_to_boundary(h3)::geometry`	`h3_cell_to_boundary_geometry(h3)`
Distance check (meters)	`ST_Distance(a.h3::geography, b.h3::geography) <= N`	`ST_DWithin(a.h3::geography, b.h3::geography, N)`
Grid distance (cells)	`h3_distance(a, b)`	`a <-> b`

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.

Ring traversal gotcha: Prefer h3_grid_ring(origin, k) for rings because it handles pentagon distortion internally. Use h3_grid_ring_unsafe(origin, k) only when fail-fast behavior or ring-walk ordering matters.

Critical v3 → v4 Renames

v3 Name (DO NOT USE)	v4 Replacement
`h3_geo_to_h3`	`h3_latlng_to_cell`
`h3_to_geo`	`h3_cell_to_latlng`
`h3_to_geo_boundary`	`h3_cell_to_boundary`
`h3_k_ring`	`h3_grid_disk`
`h3_k_ring_distances`	`h3_grid_disk_distances`
`h3_hex_ring`	`h3_grid_ring_unsafe`
`h3_line`	`h3_grid_path_cells`
`h3_distance`	`h3_grid_distance`
`h3_to_parent`	`h3_cell_to_parent`
`h3_to_children`	`h3_cell_to_children`
`h3_compact`	`h3_compact_cells`
`h3_uncompact`	`h3_uncompact_cells`
`h3_polyfill`	`h3_polygon_to_cells`
`h3_indexes_are_neighbors`	`h3_are_neighbor_cells`
`h3_hex_area`	`h3_get_hexagon_area_avg`
`h3_hex_area_km2` / `h3_hex_area_m2`	`h3_get_hexagon_area_avg(res, unit)`
`h3_edge_length_km` / `h3_edge_length_m`	`h3_get_hexagon_edge_length_avg(res, unit)`

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

name	h3-pg
description	PostgreSQL bindings for H3 hexagonal grid system. Use when working with H3 cells in Postgres, including spatial indexing, geometry/geography integration, and raster analysis.