I have a table planet_osm_roads. The table has one column ref that indicates the code of the road.

Let's say that I have a road coded 'AA1' which is represented by 5 segments (or rows in the database)

osm_id  |   ref | way(geometry)
 1         AA1    ...
 2         AA1    ...
 3         AA1    ...
 4         AA1    ... 
 5         AA1    ...

I want a new table where the 5 segments are collapsed into 1, based on their common value in ref.

Is this the correct way?

       ST_Multi(ST_Union(f.way)) as the_geom
     FROM planet_osm_roads As f
  • Yes. Is there something in the output that makes you doubt it? – John Powell Mar 9 '16 at 7:31
  • Yes, that should work. Have you tested it? – PyMapr Mar 9 '16 at 7:31
  • How long is it suppose to take on a 400k rows table? – Francesco Mar 9 '16 at 8:15
  • @Francesco, the time it takes to run the query depends on a few variables and without additional info we cannot really answer that. Have you created indexes on the table? and if you have, on which columns? – PyMapr Mar 9 '16 at 8:24
  • 1
    Test with small dataset first or add "limit 100" to your test run. – user30184 Mar 9 '16 at 21:22

Your method will work.

You can run this statement to see how many distinct records you have:

SELECT Count(DISTINCT ref) as count_of_ref
FROM planet_osm_roads As f;

And compare that count to the number of records in your new table:

SELECT Count(ref) as count_of_ref
FROM ref_roads;

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