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 ANY-INTEGER and ANY-NUMERICAL

  • 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

pgr_degree(Edges SQL , Vertex SQL, [dryrun])

RETURNS SETOF (node, degree)

OR EMTPY SET

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	TEXT	Edges SQL as described below
Vertex SQL	TEXT	Vertex SQL as described below

Optional parameters

Parameter	Type	Default	Description
dryrun	BOOLEAN	false	When true do not process and get in a NOTICE the resulting query.

Inner Queries

Edges SQL

Column	Type	Description
id	BIGINT	Identifier of the edge.

Vertex SQL

Column	Type	Description
id	BIGINT	Identifier of the first end point vertex of the edge.
in_edges	BIGINT[]	Array of identifiers of the edges that have the vertex id as first end point. When missing, out_edges must exist.
out_edges	BIGINT[]	Array of identifiers of the edges that have the vertex id as second end point. When missing, in_edges must exist.

Result columns

Column	Type	Description
node	BIGINT	Vertex identifier
degree	BIGINT	Number of edges that are incident to the vertex id

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

• Topology - Family of Functions

• pgr_extractVertices – Proposed

Indices and tables

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