我是Postgres和SQL的新手。我创建了以下脚本，从一个点到最近一行的投影点绘制一条线。它适用于5到10个小数据集，行数相同;但是，使用2,000行在60点上进行，查询大约需要12个小时。它基于下面粘贴的最近邻居函数http://www.bostongis.com/downloads/pgis_nn.txt
关于pgis_fn_nn的编辑文档可在http://www.bostongis.com/PrinterFriendly.aspx?content_name=postgis_nearest_neighbor_generic上找到
缓慢的部分是pgis_fn_nn（...）
的实现my_script.sql
-- this sql script creates a line table that connects points from a point table
-- to the projected points from the nearest line to the point of oritin
-- delete duplicate tables if they exist
DROP TABLE exploded_roads;
DROP TABLE projected_points;
DROP TABLE lines_from_centroids_to_roads;
-- create temporary exploaded lines table
CREATE TABLE exploded_roads (
the_geom geometry,
edge_id serial
);
-- insert the linestring that are not multistring
INSERT INTO exploded_roads
SELECT the_geom
FROM "StreetCenterLines"
WHERE st_geometrytype(the_geom) = 'ST_LineString';
-- insert the linestrings that need to be converted from multi string
INSERT INTO exploded_roads
SELECT the_geom
FROM (
SELECT ST_GeometryN(
the_geom,
generate_series(1, ST_NumGeometries(the_geom)))
AS the_geom
FROM "StreetCenterLines"
)
AS foo;
-- create projected points table with ids matching centroid table
CREATE TABLE projected_points (
the_geom geometry,
pid serial,
dauid int
);
-- Populate Table
-- code based on Paul Ramsey's site and Boston GIS' NN code
INSERT INTO projected_points(the_geom, dauid)
SELECT DISTINCT ON ("DAUID"::int)
(
ST_Line_Interpolate_Point(
(
SELECT the_geom
FROM exploded_roads
WHERE edge_id IN
(
SELECT nn_gid
FROM pgis_fn_nn(centroids.the_geom, 30000000, 1,10, 'exploded_roads', 'true', 'edge_id', 'the_geom')
)
),
ST_Line_Locate_Point(
exploded_roads.the_geom,
centroids.the_geom
)
)
),
(centroids."DAUID"::int)
FROM exploded_roads, fred_city_o6_da_centroids centroids;
-- Create Line tables
CREATE TABLE lines_from_centroids_to_roads (
the_geom geometry,
edge_id SERIAL
);
-- Populate Line Table
INSERT INTO lines_from_centroids_to_roads(
SELECT
ST_MakeLine( centroids.the_geom, projected_points.the_geom )
FROM projected_points, fred_city_o6_da_centroids centroids
WHERE projected_points.dauid = centroids.id
);
pgis_fn_nn
---LAST UPDATED 8/2/2007 --
CREATE OR REPLACE FUNCTION expandoverlap_metric(a geometry, b geometry, maxe double precision, maxslice double precision)
RETURNS integer AS
$BODY$
BEGIN
FOR i IN 0..maxslice LOOP
IF expand(a,maxe*i/maxslice) && b THEN
RETURN i;
END IF;
END LOOP;
RETURN 99999999;
END;
$BODY$
LANGUAGE 'plpgsql' IMMUTABLE;
CREATE TYPE pgis_nn AS
(nn_gid integer, nn_dist numeric(16,5));
CREATE OR REPLACE FUNCTION _pgis_fn_nn(geom1 geometry, distguess double precision, numnn integer, maxslices integer, lookupset varchar(150), swhere varchar(5000), sgid2field varchar(100), sgeom2field varchar(100))
RETURNS SETOF pgis_nn AS
$BODY$
DECLARE
strsql text;
rec pgis_nn;
ncollected integer;
it integer;
--NOTE: it: the iteration we are currently at
--start at the bounding box of the object (expand 0) and move up until it has collected more objects than we need or it = maxslices whichever event happens first
BEGIN
ncollected := 0; it := 0;
WHILE ncollected < numnn AND it <= maxslices LOOP
strsql := 'SELECT currentit.' || sgid2field || ', distance(ref.geom, currentit.' || sgeom2field || ') as dist FROM ' || lookupset || ' as currentit, (SELECT geometry(''' || CAST(geom1 As text) || ''') As geom) As ref WHERE ' || swhere || ' AND distance(ref.geom, currentit.' || sgeom2field || ') <= ' || CAST(distguess As varchar(200)) || ' AND expand(ref.geom, ' || CAST(distguess*it/maxslices As varchar(100)) || ') && currentit.' || sgeom2field || ' AND expandoverlap_metric(ref.geom, currentit.' || sgeom2field || ', ' || CAST(distguess As varchar(200)) || ', ' || CAST(maxslices As varchar(200)) || ') = ' || CAST(it As varchar(100)) || ' ORDER BY distance(ref.geom, currentit.' || sgeom2field || ') LIMIT ' ||
CAST((numnn - ncollected) As varchar(200));
--RAISE NOTICE 'sql: %', strsql;
FOR rec in EXECUTE (strsql) LOOP
IF ncollected < numnn THEN
ncollected := ncollected + 1;
RETURN NEXT rec;
ELSE
EXIT;
END IF;
END LOOP;
it := it + 1;
END LOOP;
END
$BODY$
LANGUAGE 'plpgsql' STABLE;
CREATE OR REPLACE FUNCTION pgis_fn_nn(geom1 geometry, distguess double precision, numnn integer, maxslices integer, lookupset varchar(150), swhere varchar(5000), sgid2field varchar(100), sgeom2field varchar(100))
RETURNS SETOF pgis_nn AS
$BODY$
SELECT * FROM _pgis_fn_nn($1,$2, $3, $4, $5, $6, $7, $8);
$BODY$
LANGUAGE 'sql' STABLE;
答案 0 :(得分：4)
我使用“最近的”函数在OpenStreetMap数据上执行pgRouting。起初我偶然发现了你提到的fn_nn函数，但访问irc.freenode.net上的#postgis irc频道帮助了我。事实证明，postgis有一些非常棒的线性函数，当它们组合在一起时，它可以回答你需要的一切！
您可以在http://postgis.refractions.net/documentation/manual-1.3/ch06.html#id2578698找到有关线性函数的更多信息，但这是我实现它的方式
select
line_interpolate_point(ways.the_geom,
line_locate_point(ways.the_geom, pnt))),')','')) as anchor_point,
-- returns the anchor point
line_locate_point(ways.the_geom, pnt) as anchor_percentage,
-- returns the percentage on the line where the anchor will
-- touch (number between 0 and 1)
CASE
WHEN line_locate_point(ways.the_geom, pnt) < 0.5 THEN ways.source
WHEN line_locate_point(ways.the_geom, pnt) > 0.5 THEN ways.target
END as node,
-- returns the nearest end node id
length_spheroid( st_line_substring(ways.the_geom,0,
line_locate_point(ways.the_geom, pnt)),
'SPHEROID[\"WGS 84\",6378137,298.257223563]' ) as length,
distance_spheroid(pnt, line_interpolate_point(ways.the_geom,
line_locate_point(ways.the_geom, pnt)),
'SPHEROID[\"WGS 84\",6378137,298.257223563]') as dist
from ways, planet_osm_line,
ST_GeomFromText('POINT(1.245 51.234)', 4326) as pnt
where ways.gid = planet_osm_line.osm_id
order by dist asc limit 1;";
希望这对你有用