Geo Searching

One of the greatest features of Manticore Search is the ability to combine full-text searching with geo location. For example a retailer can offer a search where user looks for a product and the result set can tell which is the closest shop that has the product in stock so user can go in store and pick it up. A travel site can provide results based on a search limited to a certain area and results to be sorted by the distance from a point (‘search museums near a hotel’ for example).

To perform geo searching, a document needs to contain pairs of latitude/longitude coordinates. The coordinates can be stored as float attributes. If the document has multiple locations, they can be embedded in a JSON object as float pairs.

index myrt {
...
rt_attr_float = lat
rt_attr_float = lon
...
}

The coordinates can be stored as degrees or radians.

Performing distance calculation

To find out the distance between two points the GEODIST() function can be used. GEODIST requires two pairs of coordinates as first four parameters.

A 5th parameter in a simplified JSON format can configure certain aspects of the function. By default, GEODIST expects coordinates to be in radians, but in=degrees can be added to allow using degrees at input. The coordinates for which we perform the geo distance must have same time (degrees or radians) as the ones stored in the index, otherwise results will be misleading.

The calculated distance is by default in meters, but with out option it can be transformed to kilometers, feets or miles. Lastly, by default a calculation method called adaptive is used. An alternative method based on haversine algorithm is available, however this one is slower and less precise.

The result of the function - the distance - can be used in ORDER BY clause to sort the results

SELECT *,GEODIST(40.7643929, -73.9997683, lat,lon, {in=degrees, out=miles}) AS distance FROM myindex WHERE MATCH('...') ORDER BY distance ASC, WEIGHT() DESC;

Or to limit the results to a radial area around the point:

SELECT *,GEODIST(40.7643929, -73.9997683, lat,lon, {in=degrees, out=miles}) AS distance FROM myindex WHERE MATCH('...') AND distance <1000 ORDER BY  WEIGHT(),DISTANCE ASC;

Searching in polygons

Another geo search functionality is the ability to check if a location belongs to an area. A special function will construct a polygon object which is used in another function that test if a set of coordinates belongs to it or not.

For creating the polygon two functions are available:

  • GEOPOLY2D() - creates a polygon that takes in account the Earth’s curvature
  • POLY2D() - creates a simple polygon in plain space

POLY2D can be used for geo searches if the area has sides shorter than 500km (for 3-4 sides, for polygons with more sides lower values should be considered). For areas with longer sides usage of GEOPOLY2D is mandatory for keeping results accurate. GEOPOLY2D also expects coordinates as latitude/longitude pairs in degrees, using radians will provide results in plain space (like POLY2D).

CONTAINS() expects at input a polygon and a set of coordinates and output 1 if the point is inside the polygon or 0 otherwise.

SELECT *,CONTAINS(GEOPOLY2D(40.76439, -73.9997, 42.21211, -73.999,  42.21211, -76.123, 40.76439 , -76.123), 41.5445,-74.973) AS inside FROM myindex WHERE MATCH('...') AND inside=1;