pgr_degree
– Proposed¶
pgr_degree
— For each vertex in an undirected graph, return the count of edges incident to the vertex.
Warning
Proposed functions for next mayor release.
They are not officially in the current release.
They will likely officially be part of the next mayor release:
The functions make use of ANYINTEGER and ANYNUMERICAL
Name might not change. (But still can)
Signature might not change. (But still can)
Functionality might not change. (But still can)
pgTap tests have being done. But might need more.
Documentation might need refinement.
Availability
Version 3.4.0
New proposed function
Description¶
Calculates the degree of the vertices of an undirected graph
Signatures¶
(node, degree)
 Example:
Extracting the vertex information
pgr_degree can utilize output from pgr_extractVertices or can have pgr_extractVertices embedded in the call. For decent size networks, it is best to prep your vertices table before hand and use that vertices table for pgr_degree calls.
DROP TABLE IF EXISTS tmp_edges_vertices_pgr;
NOTICE: table "tmp_edges_vertices_pgr" does not exist, skipping
DROP TABLE
CREATE TEMP TABLE tmp_edges_vertices_pgr AS
SELECT id, in_edges, out_edges
FROM pgr_extractVertices('SELECT id, geom FROM edges');
SELECT 17
SELECT * FROM pgr_degree(
$$SELECT id FROM edges$$,
$$SELECT id, in_edges, out_edges
FROM tmp_edges_vertices_pgr$$);
node  degree
+
1  1
2  1
3  2
4  1
5  1
6  3
7  4
8  3
9  1
10  3
11  4
12  3
13  1
14  1
15  2
16  3
17  2
(17 rows)
Parameters¶
Parameter 
Type 
Description 


Edges SQL as described below 


Vertex SQL as described below 
Optional parameters¶
Parameter 
Type 
Default 
Description 





Inner Queries¶
Edges SQL¶
Column 
Type 
Description 



Identifier of the edge. 
Vertex SQL¶
Column 
Type 
Description 



Identifier of the first end point vertex of the edge. 


Array of identifiers of the edges that have the vertex



Array of identifiers of the edges that have the vertex

Result columns¶
Column 
Type 
Description 



Vertex identifier 


Number of edges that are incident to the vertex 
Additional Examples¶
Degree of a sub graph¶
SELECT * FROM pgr_degree(
$$SELECT id FROM edges WHERE id < 17$$,
$$SELECT id, in_edges, out_edges
FROM pgr_extractVertices('SELECT id, geom FROM edges')$$);
node  degree
+
1  1
2  0
3  2
4  0
5  1
6  3
7  4
8  3
9  1
10  3
11  4
12  3
13  0
14  0
15  2
16  3
17  2
(17 rows)
Dry run execution¶
To get the query generated used to get the vertex information, use dryrun =>
true
.
The results can be used as base code to make a refinement based on the backend development needs.
SELECT * FROM pgr_degree(
$$SELECT id FROM edges WHERE id < 17$$,
$$SELECT id, in_edges, out_edges
FROM pgr_extractVertices('SELECT id, geom FROM edges')$$,
dryrun => true);
NOTICE:
WITH
 a sub set of edges of the graph goes here
g_edges AS (
SELECT id FROM edges WHERE id < 17
),
 sub set of vertices of the graph goes here
all_vertices AS (
SELECT id, in_edges, out_edges
FROM pgr_extractVertices('SELECT id, geom FROM edges')
),
g_vertices AS (
SELECT id,
unnest(
coalesce(in_edges::BIGINT[], '{}'::BIGINT[])

coalesce(out_edges::BIGINT[], '{}'::BIGINT[])) AS eid
FROM all_vertices
),
totals AS (
SELECT v.id, count(*)
FROM g_vertices AS v
JOIN g_edges AS e ON (e.id = eid) GROUP BY v.id
)
SELECT id::BIGINT, coalesce(count, 0)::BIGINT FROM all_vertices LEFT JOIN totals USING (id)
;
node  degree
+
(0 rows)
Degree from an existing table¶
If you have a vertices table already built using pgr_extractVertices
and want the degree of the whole graph rather than a subset, you can forgo using pgr_degree
and work with the in_edges
and out_edges
columns directly.
Dead ends¶
To get the dead ends:
SELECT id FROM vertices
WHERE array_length(in_edges  out_edges, 1) = 1;
id

1
5
9
13
14
2
4
(7 rows)
That information is correct, for example, when the dead end is on the limit of the imported graph.
Visually node \(4\) looks to be as start/ending of 3 edges, but it is not.
Is that correct?
Is there such a small curb:
That does not allow a vehicle to use that visual intersection?
Is the application for pedestrians and therefore the pedestrian can easily walk on the small curb?
Is the application for the electricity and the electrical lines than can easily be extended on top of the small curb?
Is there a big cliff and from eagles view look like the dead end is close to the segment?
When there are many dead ends, to speed up, the Contraction  Family of functions functions can be used to divide the problem.
Linear edges¶
To get the linear edges:
SELECT id FROM vertices
WHERE array_length(in_edges  out_edges, 1) = 2;
id

3
15
17
(3 rows)
This information is correct, for example, when the application is taking into account speed bumps, stop signals.
When there are many linear edges, to speed up, the Contraction  Family of functions functions can be used to divide the problem.
See Also¶
Indices and tables