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The earthdistance extension in PostgreSQL provides functions for calculating great-circle distances between points on Earth. It is useful for applications that require geospatial distance calculations, such as location-based services and mapping applications. Your Nile database arrives with earthdistance extension and its dependency cube already enabled, so there’s no need to run create extension.

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Creating and Populating locations Table

Before performing distance calculations, let’s create a sample table to store latitude and longitude values: 
CREATE TABLE locations (
    id SERIAL PRIMARY KEY,
    name TEXT,
    latitude DOUBLE PRECISION,
    longitude DOUBLE PRECISION
);

INSERT INTO locations (name, latitude, longitude) VALUES
    ('San Francisco', 37.7749, -122.4194),
    ('New York', 40.7128, -74.0060),
    ('Los Angeles', 34.0522, -118.2437),
    ('Chicago', 41.8781, -87.6298),
    ('Miami', 25.7617, -80.1918);

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Calculating Distance Between Two Points

To compute the great-circle distance between two points (given in latitude and longitude in degrees), use the earth_distance() function: 
SELECT a.name AS location_a, b.name AS location_b, 
       earth_distance(ll_to_earth(a.latitude, a.longitude), ll_to_earth(b.latitude, b.longitude)) AS distance_meters
FROM locations a, locations b
WHERE a.name = 'San Francisco' AND b.name = 'New York';
This returns the approximate distance in meters between San Francisco and New York.

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Finding Locations Within a Given Radius

To find all locations within 1000 km of San Francisco: 
SELECT name, earth_distance(ll_to_earth(37.7749, -122.4194), ll_to_earth(latitude, longitude)) AS distance_meters
FROM locations
WHERE earth_distance(ll_to_earth(37.7749, -122.4194), ll_to_earth(latitude, longitude)) < 1000000;

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Limitations

  • earthdistance assumes a spherical Earth model, which may introduce minor inaccuracies.
  • Distance calculations are approximate and may not be suitable for high-precision geospatial applications.
  • Requires both cube and earthdistance extensions to be installed.

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Removing an Index

If you need to remove the spatial index: 
DROP INDEX locations_earth_idx;

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Conclusion

The earthdistance extension in PostgreSQL simplifies great-circle distance calculations for geographic coordinates. It is useful for applications needing fast location-based searches and distance queries. For more details, refer to the PostgreSQL documentation.