design-postgis-tables

What this skill does

# PostGIS Spatial Table Design

## Before You Start (5 Questions)

1. What is the geographic scope (single city/region vs global)?
2. What are your primary query patterns (within-radius, bbox, intersects, nearest-neighbor)?
3. What units do you need for distance/area (meters vs CRS units), and how accurate must they be?
4. What is the expected scale (rows, write rate), and is the data mostly append-only?
5. Do you need 3D (Z) or measures (M), or is 2D enough?

**SQL injection note:** When turning these patterns into application code, use parameterized queries for user-provided values (WKT/WKB, coordinates, IDs, radii). Avoid string-concatenating untrusted input into SQL; for dynamic identifiers, use safe identifier quoting/whitelisting.

## Core Rules

- **Always use PostGIS geometry/geography types** instead of PostgreSQL's built-in geometric types (`POINT`, `LINE`, `POLYGON`, `CIRCLE`). PostGIS types provide true spatial capabilities.
- **Choose between GEOMETRY and GEOGRAPHY** based on your use case: GEOMETRY for projected/local data with Cartesian math; GEOGRAPHY for global data requiring accurate spherical calculations.
- **Always specify SRID** (Spatial Reference Identifier) when creating geometry columns. Use `4326` (WGS84) for GPS/global data, appropriate local projections for regional data.
- **Create spatial indexes** on all geometry/geography columns using GiST (default). Consider BRIN only for very large **GEOMETRY** tables where rows are naturally ordered on disk and you can tolerate coarser filtering.
- **Use constraint-based type enforcement** with `GEOMETRY(type, SRID)` syntax to ensure data integrity.

## Geometry vs Geography

### When to Use GEOMETRY

- **Local/regional data** within a single coordinate system
- **Projected coordinates** (meters, feet) for accurate area/distance calculations
- **Complex spatial operations** (buffering, unions, intersections)
- **Performance-critical queries** (Cartesian math is faster)
- **Data already in a projected CRS** (UTM, State Plane, etc.)

```sql
-- Regional data with projected coordinates (UTM Zone 10N for California)
CREATE TABLE local_parcels (
    id BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
    parcel_number TEXT NOT NULL,
    boundary GEOMETRY(POLYGON, 26910),  -- UTM Zone 10N (meters)
    area_sqm DOUBLE PRECISION GENERATED ALWAYS AS (ST_Area(boundary)) STORED
);
```

### When to Use GEOGRAPHY

- **Global data** spanning multiple continents/hemispheres
- **GPS coordinates** (latitude/longitude in decimal degrees)
- **Accurate distance calculations** on Earth's surface (great circle)
- **Simple spatial operations** (distance, containment)
- **Data from GPS devices, geocoding services, or web maps**

```sql
-- Global data with geodetic calculations
CREATE TABLE global_offices (
    id BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
    name TEXT NOT NULL,
    city TEXT NOT NULL,
    location GEOGRAPHY(POINT, 4326)  -- WGS84 (lat/lon)
);

-- Distance in meters (accurate spherical calculation)
SELECT
    a.name AS office_a,
    b.name AS office_b,
    ST_Distance(a.location, b.location) / 1000 AS distance_km
FROM global_offices a
CROSS JOIN global_offices b
WHERE a.id < b.id;
```

### Comparison Table

| Aspect            | GEOMETRY                              | GEOGRAPHY                 |
| ----------------- | ------------------------------------- | ------------------------- |
| Coordinate system | Any SRID (projected or geodetic)      | WGS84 (SRID 4326) only    |
| Distance units    | CRS units (degrees, meters, feet)     | Meters (always)           |
| Distance accuracy | Depends on projection                 | True spheroidal distance  |
| Area accuracy     | Accurate in projected CRS             | Accurate on sphere        |
| Function support  | Full (300+ functions)                 | Limited (~40 functions)   |
| Performance       | Faster (Cartesian math)               | Slower (spherical math)   |
| Index type        | GiST, BRIN, SP-GiST                   | GiST only                 |
| Best for          | Regional/local data, complex analysis | Global data, GPS tracking |

## Geometry Types

### Point Types

```sql
-- Single location (stores, sensors, events)
location GEOMETRY(POINT, 4326)

-- Multiple discrete locations (multi-branch business)
locations GEOMETRY(MULTIPOINT, 4326)

-- 3D point with elevation
location_3d GEOMETRY(POINTZ, 4326)

-- Point with measure value (linear referencing)
location_m GEOMETRY(POINTM, 4326)
```

**Use POINT for:** Store locations, sensor positions, event coordinates, addresses, POIs
**Use MULTIPOINT for:** Multiple related locations stored as single feature

### Line Types

```sql
-- Single path (road segment, river, route)
path GEOMETRY(LINESTRING, 4326)

-- Multiple paths (road network, transit lines)
network GEOMETRY(MULTILINESTRING, 4326)

-- 3D line with elevation profile
trail_3d GEOMETRY(LINESTRINGZ, 4326)
```

**Use LINESTRING for:** Roads, rivers, pipelines, GPS tracks, routes
**Use MULTILINESTRING for:** Disconnected road segments, river systems

### Polygon Types

```sql
-- Single area (parcel, building footprint, zone)
boundary GEOMETRY(POLYGON, 4326)

-- Multiple areas (archipelago, fragmented habitat)
territories GEOMETRY(MULTIPOLYGON, 4326)

-- 3D polygon (building with height)
footprint_3d GEOMETRY(POLYGONZ, 4326)
```

**Use POLYGON for:** Property boundaries, administrative areas, service zones
**Use MULTIPOLYGON for:** Countries with islands, fragmented regions

### Generic Types

```sql
-- Any geometry type (flexible schema)
geom GEOMETRY(GEOMETRY, 4326)

-- Collection of mixed types
features GEOMETRY(GEOMETRYCOLLECTION, 4326)
```

**Use GEOMETRY for:** Flexible schemas accepting multiple types
**Avoid GEOMETRYCOLLECTION:** Prefer homogeneous types for better indexing

## Coordinate Systems (SRID)

### Common SRIDs

| SRID        | Name              | Use Case                     | Units   |
| ----------- | ----------------- | ---------------------------- | ------- |
| 4326        | WGS84             | GPS, global data, web maps   | Degrees |
| 3857        | Web Mercator      | Web map tiles (display only) | Meters  |
| 26910-26919 | UTM Zones (US)    | Regional analysis            | Meters  |
| 32601-32660 | UTM Zones (North) | Regional analysis            | Meters  |
| 32701-32760 | UTM Zones (South) | Regional analysis            | Meters  |

### SRID Best Practices

- **Store in WGS84 (4326)** for interoperability and GPS data
- **Transform to projected CRS** for accurate measurements
- **Never mix SRIDs** in spatial operations without explicit transformation
- **Use appropriate local CRS** for area/distance calculations requiring high precision

```sql
-- Store in WGS84, calculate in UTM
CREATE TABLE survey_points (
    id BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
    location GEOMETRY(POINT, 4326),  -- Storage: WGS84
    CONSTRAINT valid_location CHECK (ST_IsValid(location))
);

-- Calculate distance in meters using UTM projection
SELECT
    a.id AS point_a,
    b.id AS point_b,
    ST_Distance(
        ST_Transform(a.location, 26910),  -- Transform to UTM
        ST_Transform(b.location, 26910)
    ) AS distance_meters
FROM survey_points a
CROSS JOIN survey_points b
WHERE a.id < b.id;
```

## Spatial Indexing

### GiST Index (Default)

Most versatile spatial index. Use for all geometry/geography columns.

```sql
-- Geometry (most common)
CREATE INDEX idx_your_table_geom_gist ON your_table_name USING GIST (geom);

-- Geography (GiST is the supported option)
CREATE INDEX idx_your_table_geog_gist ON your_table_name USING GIST (geog);

-- Analyze after index creation
VACUUM ANALYZE your_table_name;
```

**Supports:** All spatial operators (`&&`, `@>`, `<@`, `~=`, `<->`)
**Best for:** General-purpose spatial queries, mixed query patterns

### BRIN Index

Block Range Index for very large, naturally ordered datasets.

```sql
-- BRIN for very large, append-only GEOMETRY tables (geography uses GiST)
CREAT