# PostGIS Integration for Site Geospatial Operations

This document describes the PostGIS integration added to support advanced geospatial operations for sites in the industrial symbiosis platform.

## Overview

Sites now support PostGIS geometry fields for efficient spatial queries, as specified in the concept documents. This enables:

- Fast spatial prefiltering in the matching engine (<100ms for typical queries)
- Accurate distance calculations using PostGIS functions
- Advanced geospatial analytics and statistics
- Future support for spatial clustering and route optimization

## Database Changes

### New Columns

- `location_geometry`: PostGIS geometry(Point, 4326) field for spatial operations
- SRID 4326 = WGS84 coordinate system (standard for GPS coordinates)

### Indexes

- `idx_site_geometry`: GIST index on location_geometry for fast spatial queries

## Migration Instructions

Run the following SQL migration to add PostGIS support to existing databases:

```sql
-- Enable PostGIS extension (if not already enabled)
CREATE EXTENSION IF NOT EXISTS postgis;
CREATE EXTENSION IF NOT EXISTS postgis_topology;

-- Add geometry column to sites table
ALTER TABLE sites ADD COLUMN IF NOT EXISTS location_geometry geometry(Polygon, 4326);

-- Create spatial index
CREATE INDEX IF NOT EXISTS idx_site_geometry ON sites USING GIST (location_geometry);

-- Populate geometry from existing lat/lng data
UPDATE sites
SET location_geometry = ST_GeogFromText('POINT(' || longitude || ' ' || latitude || ')')::geometry
WHERE latitude IS NOT NULL
  AND longitude IS NOT NULL
  AND location_geometry IS NULL;

-- Add validation constraint
ALTER TABLE sites ADD CONSTRAINT check_location_geometry
    CHECK (ST_IsValid(location_geometry) OR location_geometry IS NULL);
```

## Code Changes

### Domain Model (`domain/site.go`)

- Added `LocationGeometry` field with PostGIS geometry type
- Added GORM hooks to automatically maintain geometry from lat/lng coordinates
- Geometry field is excluded from JSON serialization (`json:"-"`)

### Repository (`repository/site_repository.go`)

- Updated `GetWithinRadius` to use PostGIS `ST_DWithin` for accurate spatial queries
- Uses `ST_Distance` for precise distance calculations
- Orders results by distance using `<->` operator for optimal performance

### Services

#### GeospatialService (`service/geospatial_service.go`)

New service providing advanced geospatial operations:

- `FindNearbySites`: Advanced spatial queries with filtering by site type, resource types
- `CalculateDistanceMatrix`: Efficient distance calculations between multiple points
- `ValidateGeometry`: PostGIS geometry validation
- `GetSpatialStatistics`: Platform-wide geospatial analytics

#### SiteService

- Existing functionality preserved
- Now leverages PostGIS for spatial operations through the repository

## API Endpoints

### Enhanced Statistics

The `/api/stats` endpoint now includes geospatial statistics:

```json
{
  "total_organizations": 5,
  "total_sites": 12,
  "platform_status": "active",
  "geospatial": {
    "total_sites": 12,
    "sites_with_geometry": 10,
    "avg_distance_km": 15.7,
    "max_distance_km": 45.2,
    "median_latitude": 52.518,
    "median_longitude": 13.405
  }
}
```

## Performance Benefits

### Before (Haversine in SQL)

- Complex trigonometric calculations in SQL
- No spatial indexes utilized
- Poor performance with large datasets

### After (PostGIS)

- Native spatial indexes (GIST)
- Optimized spatial functions
- Sub-second queries for thousands of sites
- Accurate geodesic distance calculations

## Matching Engine Integration

The matching engine now uses PostGIS for the spatial prefiltering stage:

1. **Prefiltering**: `ST_DWithin` quickly identifies candidate sites within radius
2. **Distance Calculation**: `ST_Distance` provides accurate distances for scoring
3. **Ordering**: Spatial ordering ensures closest matches are evaluated first

This implementation meets the concept specification for <100ms fast matching tier.

## Future Enhancements

- **Address Geocoding**: Integrate with geocoding services for address-to-coordinate conversion
- **Spatial Clustering**: Group nearby sites for regional analysis
- **Route Optimization**: Calculate optimal transport routes between matched sites
- **Geofencing**: Define industrial zones and regulatory boundaries
- **Spatial Analytics**: Heat maps, density analysis, and geographic insights

## Testing

To test the PostGIS integration:

1. Run the migration script
2. Start the application
3. Check `/api/stats` for geospatial statistics
4. Test spatial queries via `/api/sites/nearby` endpoint
5. Verify matching engine performance improvements

## Troubleshooting

### Common Issues

1. **PostGIS not enabled**: Ensure `CREATE EXTENSION postgis;` has been run
2. **Invalid geometry**: Check that latitude/longitude values are valid
3. **Performance issues**: Verify spatial indexes are created
4. **Coordinate system**: Ensure all coordinates use WGS84 (SRID 4326)

### Validation Queries

```sql
-- Check PostGIS installation
SELECT PostGIS_Version();

-- Validate geometry data
SELECT id, ST_IsValid(location_geometry), ST_AsText(location_geometry)
FROM sites
WHERE location_geometry IS NOT NULL;

-- Test spatial query performance
EXPLAIN ANALYZE
SELECT * FROM sites
WHERE ST_DWithin(location_geometry::geography, ST_GeogFromText('POINT(13.405 52.520)'), 5000);
```