[pgrouting-users] Reduce the graph

[Aurélien FILEZ]

I'll try starting from this.

Thank you very much

On Sat, Mar 10, 2012 at 3:51 PM, Stephen Woodbridge <woodbri at swoodbridge.com
> wrote:

> On 3/10/2012 2:20 AM, Aurélien FILEZ wrote:
>
>> Hi,
>>
>> This table is build with lines I decompose in segments of 2 points.
>>
>> Maybe it is better to act from these full lines, searching points used
>> by more than one line ?
>>
>
> Yes, but the question that I am trying to answer is How are you going to
> do the efficiently?
>
> I just noticed that you already have v1 and v2 assigned, so you can do
> something like this:
>
> create table vertices_tmp as
> select id, count(*) as cnt from
>   (select v1 as id from edge union all select v2 as id from edge) as foo
> group by id;
>
> Now to find all the the nodes where cnt=2
>
> select id from vertices_tmp where cnt=2;
>
> And you write a stored procedure to merge the two edges.
>
> -Steve
>
>  Thanks,
>> Kin
>>
>> On Fri, Mar 9, 2012 at 5:48 PM, Stephen Woodbridge
>> <woodbri at swoodbridge.com <mailto:woodbri at swoodbridge.**com<woodbri at swoodbridge.com>>>
>> wrote:
>>
>>    On 3/9/2012 11:39 AM, Aurélien FILEZ wrote:
>>
>>        Hi all,
>>
>>        My edge table has some useless nodes. For example :
>>
>>        edge_id    v1      v2      the_geom
>>        1              1        2      LINESTRING(x1 y1, x2 y2)
>>        2              2        3      LINESTRING(x2 y2, x3 y3)
>>
>>        But the vertex 2 is no used in any other edge.
>>
>>        So I would like reduce my graph to transforme this previous
>>        example to :
>>        edge_id    v1      v2      the_geom
>>        1              1        3      LINESTRING(x1 y1, x2 y2, x3 y3)
>>
>>        Is there is a way to do that ?
>>
>>        Thank you all :)
>>
>>
>>    There is no automatic way to do this. I have done a lot of graph
>>    analysis by adding a cnt column to the vertices_tmp column and then
>>    updating it to the count of edges connected to that vertex.
>>
>>    Then you can do things like look for:
>>
>>    deadends - cnt=1
>>    connected lines - cnt=2
>>
>>    You could write a stored procedure to that the connected lines, join
>>    them together, and update the relevant tables to reflect the new
>>    topology.
>>
>>    -Steve
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>>
>>
>>
>>
>>
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