项目中用到了postgreSQL中的earthdistance()函数功能计算地球上两点之间的距离，中文的资料太少了，我找到了一篇 英文的、讲的很好的文章 ，特此翻译，希望能够帮助到以后用到earthdistance的同学。
做一个GEO应用从来都不是一件容易的事。但是用一些身边的开源项目就可以在几分钟内轻松解决这个问题。 PostgreSQL有许多特性。是我的首选，它能够把数据库平台提升到另一个层次。
计算2个坐标之间的距离，我们要用到 earthdistance(lltoearth($latlngcube), lltoearth($latlng_cube)) 这个函数。 earthdistance()函数接受2组坐标值，返回值一个以米为单位的的数值。
【译者注】大圆距离（Great circle disstance）指的是从球面的一点A出发到达球面上另一点B，所经过的最短路径的长度。一般说来，球面上任意两点A和B都可以与球心确定唯一的大圆，这个大圆被称为黎曼圆，而在大圆上连接这两点的较短的一条弧的长度就是大圆距离。如果想了解更多，请看wiki： 大圆距离
SELECT events.id, events.name FROM events WHERE earthbox({currentuserlat}, {currentuserlng}, {radiusinmetres}) @> llto_earth(events.lat, events.lng);
我的应用比较简单，所以我把经纬度（lat和lng）都设成了double类型。这使得我用Node.js开发起来更加快速，而不用再去自己定制针对GIST类型的解决方案。
/*
* postgreSQL之earthdistance学习笔记
* author: wusuopubupt
* date: 2013-03-31
*/
/*创建表*/
CREATE TABLE picture (
id serial PRIMARY KEY ,
p_uid char(12) NOT NULL,
p_key char(23) NOT NULL,
lat real not null,
lng real NOT NULL,
up int NOT NULL,
down int NOT NULL,
ip varchar(15) DEFAULT NULL,
address varchar(256) DEFAULT NULL
);
/*插入记录*/
INSERT INTO picture(p_uid, p_key, lat, lng, up, down, ip, address)
VALUES('aaaabbbbcccc', '2014032008164023279.png', 40.043945, 116.413668, 0, 0, '', '');
/*插入记录*/
INSERT INTO picture(p_uid, p_key, lat, lng, up, down, ip, address)
VALUES('xxxxccccmmmm', '2014032008164023111.png', 40.067183, 116.415230, 0, 0, '', '');
/*选择记录*/
SELECT * FROM picture;
/*更新记录*/
UPDATE picture SET address='LiShuiqiao' WHERE id=1;
UPDATE picture SET address='TianTongyuan' WHERE id=2;
/*对经纬度列创建索引*/
CREATE INDEX ll_idx on picture USING gist(ll_to_earth(lat, lng));
/*根据半径（1000米）选择记录*/
SELECT * FROM picture where earth_box(ll_to_earth(40.059286,116.418773),1000) @> ll_to_earth(picture.lat, picture.lng);
/*选择距离当前用户的距离*/
SELECT picture.id, earth_distance(ll_to_earth(picture.lat, picture.lng), ll_to_earth(40.059286,116.418773))
AS dis FROM picture
ORDER BY dis ASC;
/*
* 以下内容是网上的一篇教程
* 地址：http://www.cse.iitb.ac.in/dbms/Data/Courses/CS631/PostgreSQL-Resources/postgresql-9.2.4/contrib/earthdistance/expected/earthdistance.out
*/
--
-- Test earthdistance extension
--
-- In this file we also do some testing of extension create/drop scenarios.
-- That's really exercising the core database's dependency logic, so ideally
-- we'd do it in the core regression tests, but we can't for lack of suitable
-- guaranteed-available extensions. earthdistance is a good test case because
-- it has a dependency on the cube extension.
--
CREATE EXTENSION earthdistance; -- fail, must install cube first
ERROR: required extension "cube" is not installed
CREATE EXTENSION cube;
CREATE EXTENSION earthdistance;
--
-- The radius of the Earth we are using.
--
SELECT earth()::numeric(20,5);
earth
---------------
6378168.00000
(1 row)
--
-- Convert straight line distances to great circle distances.把直线距离转成大圆距离
--
SELECT (pi()*earth())::numeric(20,5);
numeric
----------------
20037605.73216
(1 row)
SELECT sec_to_gc(0)::numeric(20,5);
sec_to_gc
-----------
0.00000
(1 row)
--
-- Convert great circle distances to straight line distances.
--
SELECT gc_to_sec(0)::numeric(20,5);
gc_to_sec
-----------
0.00000
(1 row)
SELECT gc_to_sec(sec_to_gc(2*earth()))::numeric(20,5);
gc_to_sec
----------------
12756336.00000
(1 row)
--
-- Set coordinates using latitude and longitude.
-- Extract each coordinate separately so we can round them.
--
SELECT cube_ll_coord(ll_to_earth(0,0),1)::numeric(20,5),
cube_ll_coord(ll_to_earth(0,0),2)::numeric(20,5),
cube_ll_coord(ll_to_earth(0,0),3)::numeric(20,5);
cube_ll_coord | cube_ll_coord | cube_ll_coord
---------------+---------------+---------------
6378168.00000 | 0.00000 | 0.00000
(1 row)
SELECT cube_ll_coord(ll_to_earth(360,360),1)::numeric(20,5),
cube_ll_coord(ll_to_earth(360,360),2)::numeric(20,5),
cube_ll_coord(ll_to_earth(360,360),3)::numeric(20,5);
cube_ll_coord | cube_ll_coord | cube_ll_coord
---------------+---------------+---------------
6378168.00000 | 0.00000 | 0.00000
(1 row)
--
-- Test getting the latitude of a location.
--
SELECT latitude(ll_to_earth(0,0))::numeric(20,10);
latitude
--------------
0.0000000000
(1 row)
SELECT latitude(ll_to_earth(45,0))::numeric(20,10);
latitude
---------------
45.0000000000
(1 row)
--
-- Test getting the longitude of a location.
--
SELECT longitude(ll_to_earth(0,0))::numeric(20,10);
longitude
--------------
0.0000000000
(1 row)
SELECT longitude(ll_to_earth(45,0))::numeric(20,10);
longitude
--------------
0.0000000000
(1 row)
--
-- For the distance tests the following is some real life data.
--
-- Chicago has a latitude of 41.8 and a longitude of 87.6.
-- Albuquerque has a latitude of 35.1 and a longitude of 106.7.
-- (Note that latitude and longitude are specified differently
-- in the cube based functions than for the point based functions.)
--
--
-- Test getting the distance between two points using earth_distance.
--
SELECT earth_distance(ll_to_earth(0,0),ll_to_earth(0,0))::numeric(20,5);
earth_distance
----------------
0.00000
(1 row)
SELECT earth_distance(ll_to_earth(0,0),ll_to_earth(0,180))::numeric(20,5);
earth_distance
----------------
20037605.73216
(1 row)
--
-- Test getting the distance between two points using geo_distance.
--
SELECT geo_distance('(0,0)'::point,'(0,0)'::point)::numeric(20,5);
geo_distance
--------------
0.00000
(1 row)
SELECT geo_distance('(0,0)'::point,'(180,0)'::point)::numeric(20,5);
geo_distance
--------------
12436.77274
(1 row)
--
-- Test getting the distance between two points using the @> operator.
--
SELECT ('(0,0)'::point @> '(0,0)'::point)::numeric(20,5);
numeric
---------
0.00000
(1 row)
SELECT ('(0,0)'::point @> '(180,0)'::point)::numeric(20,5);
numeric
-------------
12436.77274
(1 row)
--
-- Test for points that should be in bounding boxes.
--
SELECT earth_box(ll_to_earth(0,0),
earth_distance(ll_to_earth(0,0),ll_to_earth(0,1))*1.00001) @>
ll_to_earth(0,1);
?column?
----------
t
(1 row)
SELECT earth_box(ll_to_earth(0,0),
earth_distance(ll_to_earth(0,0),ll_to_earth(0,0.1))*1.00001) @>
ll_to_earth(0,0.1);
?column?
----------
t
(1 row)
--
-- Test for points that shouldn't be in bounding boxes. Note that we need
-- to make points way outside, since some points close may be in the box
-- but further away than the distance we are testing.
--
SELECT earth_box(ll_to_earth(0,0),
earth_distance(ll_to_earth(0,0),ll_to_earth(0,1))*.57735) @>
ll_to_earth(0,1);
?column?
----------
f
(1 row)
SELECT earth_box(ll_to_earth(0,0),
earth_distance(ll_to_earth(0,0),ll_to_earth(0,0.1))*.57735) @>
ll_to_earth(0,0.1);
?column?
----------
f
(1 row)
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