pgr_johnson

pgr_johnson - Returns the sum of the costs of the shortest path for each pair of nodes in the graph using Floyd-Warshall algorithm.

_images/boost-inside.jpeg

Boost Graph Inside

Availability

  • Version 2.2.0

    • Signature change

    • Old signature no longer supported

  • Version 2.0.0

    • Official function

Description

The Johnson algorithm, is a good choice to calculate the sum of the costs of the shortest path for each pair of nodes in the graph, for sparse graphs. It usees the Boost’s implementation which runs in \(O(V E \log V)\) time,

The main characteristics are:

  • It does not return a path.

  • Returns the sum of the costs of the shortest path for each pair of nodes in the graph.

  • Process is done only on edges with positive costs.

  • Boost returns a \(V \times V\) matrix, where the infinity values. Represent the distance between vertices for which there is no path.

    • We return only the non infinity values in form of a set of (start_vid, end_vid, agg_cost).

  • Let be the case the values returned are stored in a table, so the unique index would be the pair: (start_vid, end_vid).

  • For the undirected graph, the results are symmetric.

    • The agg_cost of (u, v) is the same as for (v, u).

  • When start_vid = end_vid, the agg_cost = 0.

  • Recommended, use a bounding box of no more than 3500 edges.

Signatures

Summary

pgr johnson(Edges SQL [, directed])
RETURNS SET OF (start_vid, end_vid, agg_cost)
OR EMPTY SET
Example:

For a directed subgraph with edges \(\{1, 2, 3, 4\}\).

SELECT * FROM pgr_johnson(
  'SELECT source, target, cost FROM edges
  WHERE id < 5'
) ORDER BY start_vid, end_vid;
 start_vid | end_vid | agg_cost
-----------+---------+----------
         5 |       6 |        1
         5 |       7 |        2
         6 |       7 |        1
(3 rows)

Parameters

Parameter

Type

Default

Description

Edges SQL

TEXT

Edges SQL as described below.

Optional parameters

Column

Type

Default

Description

directed

BOOLEAN

true

  • When true the graph is considered Directed

  • When false the graph is considered as Undirected.

Inner Queries

Edges SQL

Column

Type

Default

Description

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)

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 (start_vid, end_vid, agg_cost)

Set of (start_vid, end_vid, agg_cost)

Column

Type

Description

start_vid

BIGINT

Identifier of the starting vertex.

end_vid

BIGINT

Identifier of the ending vertex.

agg_cost

FLOAT

Aggregate cost from start_vid to end_vid.

See Also

Indices and tables