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I have to build a street network with Postgis that will be used with pgrouting.

I have two tables: 1) the_network (182 lines): is a MULTILINESTRING table. Having columns road_name (varchar) / road_length (numeric)

(numeric) 2) buildings (104 lines): is a POINT table. Having building_id (integer)

The following code creates lines between buildings and the network. The 104 created lines are stored in

buildings_to_network_lines_table which is GEOMETRY type.

SELECT ST_ShortestLine(a.geom, ST_Collect(b.geom)) AS geom, ST_Distance(a.geom, ST_Collect(b.geom)) AS shape_length INTO buildings_to_network_lines_table
FROM  buildings a, the_network b
WHERE ST_Dwithin (a.geom, b.geom, 1000)
group by a.gid, a.geom;

How to create a new table containing the black lines (the_network) and the red lines (buildings_to_network_lines_table)

Then i just want to create one table named the_network_full that contains all the lines of the_network and buildings_to_network_lines_table tables. I've tried to achieve this by a ST_UNION function :

SELECT ST_Union(a.geom, b.geom) AS the_network_full
FROM  the_network a, buildings_to_network_lines_table b;

enter image description here

The results looks good but unfortunatelly the resulting the_network_full table (type GEOMETRY) contains 18'928 lines (182 * 104) instead of 182+104 = 286 lines. Any idea of what I am doing wrong ? How would you do to merge these two tables into a new one ?

Any help will be greatly appreciated, Theo

  • Did you want a geometrical union? Seems like you want the union of tables: SELECT geom AS the_network_full FROM the_network UNION SELECT geom AS the_network_full FROM buildings_to_network_lines_table; – cag Apr 9 '14 at 12:14
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You implicitly CROSS JOINed the tables so you got a cartesian product. Since you don't want to merge the geometries together you should use the UNION ALL which is an entirely different construct:

SELECT a.geom
FROM  the_network a
UNION ALL
SELECT b.geom
FROM buildings_to_network_lines_table b

Now that you have a correct SELECT query you can use it in INSERT ... SELECT to put the data in new table.

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