pgr_prim

pgr_prim — Minimum spanning forest of graph using Prim algorithm.

_images/boost-inside.jpeg

Boost Graph Inside

Availability

Support

Description

This algorithm finds the minimum spanning forest in a possibly disconnected graph using Prim’s algorithm.

The main characteristics are:

  • It’s implementation is only on undirected graph.
  • Process is done only on edges with positive costs.
  • When the graph is connected
    • The resulting edges make up a tree
  • When the graph is not connected,
    • Finds a minimum spanning tree for each connected component.
    • The resulting edges make up a forest.
  • Prim’s running time: \(O(E*log V)\)
  • EMPTY SET is returned when there are no edges in the graph.

Signatures

Summary

pgr_prim(edges_sql)

RETURNS SET OF (edge, cost)
OR EMPTY SET
Example:Minimum Spanning Forest of a subgraph 
SELECT edge, cost FROM pgr_prim(
    'SELECT id, source, target, cost, reverse_cost FROM edge_table WHERE id < 14'
) ORDER BY edge;
 edge | cost
------+------
    1 |    1
    2 |    1
    3 |    1
    4 |    1
    5 |    1
    6 |    1
    7 |    1
    9 |    1
   10 |    1
   11 |    1
   13 |    1
(11 rows)

Parameters

Parameter Type Description
Edges SQL TEXT SQL query described in Inner query.

Inner query

Column Type Default Description
id ANY-INTEGER   Identifier of the edge.
source ANY-INTEGER   Identifier of the first end point vertex of the edge.
target ANY-INTEGER   Identifier of the second end point vertex of the edge.
cost ANY-NUMERICAL  

Weight of the edge (source, target)

  • When negative: edge (source, target) does not exist, therefore it’s not part of the graph.
reverse_cost ANY-NUMERICAL -1

Weight of the edge (target, source),

  • When negative: edge (target, source) does not exist, therefore it’s not part of the graph.

Where:

ANY-INTEGER:SMALLINT, INTEGER, BIGINT
ANY-NUMERICAL:SMALLINT, INTEGER, BIGINT, REAL, FLOAT

Result Columns

Returns SET OF (edge, cost)

Column Type Description
edge BIGINT Identifier of the edge.
cost FLOAT Cost to traverse the edge.