PostGIS 1.5.8 Manual - Fedora Project Packages GIT repositories

PostGIS 1.5.8 Manual187 / 310NoteThis function call will automatically include a bounding box comparison that will make use of any indexes that areavailable on the geometries.NotePrior to 1.3, ST_Expand was commonly used in conjunction with && and ST_Distance to achieve the same effect andin pre-1.3.4 this function was basically short-hand for that construct. From 1.3.4, ST_DWithin uses a more short-circuitdistance function which should make it more efficient than prior versions for larger buffer regions.This method implements the OpenGIS Simple Features Implementation Specification for SQL 1.1.Availability: 1.5.0 support for geography was introducedExamples--Find the nearest hospital to each school--that is within 3000 units of the school.-- We do an ST_DWithin search to utilize indexes to limit our search list-- that the non-indexable ST_Distance needs to process--If the units of the spatial reference is meters then units would be metersSELECT DISTINCT ON (s.gid) s.gid, s.school_name, s.the_geom, h.hospital_nameFROM schools sLEFT JOIN hospitals h ON ST_DWithin(s.the_geom, h.the_geom, 3000)ORDER BY s.gid, ST_Distance(s.the_geom, h.the_geom);--The schools with no close hospitals--Find all schools with no hospital within 3000 units--away from the school. Units is in units of spatial ref (e.g. meters, feet, degrees)SELECT s.gid, s.school_nameFROM schools sLEFT JOIN hospitals h ON ST_DWithin(s.the_geom, h.the_geom, 3000)WHERE h.gid IS NULL;See AlsoST_Distance, ST_Expand7.8.19 ST_EqualsST_Equals — Returns true if the given geometries represent the same geometry. Directionality is ignored.Synopsisboolean ST_Equals(geometry A, geometry B);DescriptionReturns TRUE if the given Geometries are "spatially equal". Use this for a ’better’ answer than ’=’. Note by spatially equal wemean ST_Within(A,B) = true and ST_Within(B,A) = true and also mean ordering of points can be different but represent the samegeometry structure. To verify the order of points is consistent, use ST_OrderingEquals (it must be noted ST_OrderingEquals isa little more stringent than simply verifying order of points are the same).