pgr_edmondsKarp

pgr_edmondsKarp — Calculates the flow on the graph edges that maximizes the flow from the sources to the targets using Edmonds Karp Algorithm.

Boost Graph Inside

Availability

• Version 3.2.0

  • New proposed signature

    • pgr_edmondsKarp (Combinations)

• Version 3.0.0

  • Official function

• Version 2.5.0

  • Renamed from pgr_maxFlowEdmondsKarp

  • Proposed function

• Version 2.3.0

  • New Experimental function

Description

The main characteristics are:

• The graph is directed.

• Process is done only on edges with positive capacities.

• When the maximum flow is 0 then there is no flow and EMPTY SET is returned.

  • There is no flow when source has the same vaule as target.

• Any duplicated values in source or target are ignored.

• Calculates the flow/residual capacity for each edge. In the output

  • Edges with zero flow are omitted.

• Creates

  • a super source and edges from it to all the sources,

  • a super target and edges from it to all the targetss.

• The maximum flow through the graph is guaranteed to be the value returned by pgr_maxFlow when executed with the same parameters and can be calculated:

  • By aggregation of the outgoing flow from the sources

  • By aggregation of the incoming flow to the targets

• Running time: \(O( V * E ^ 2)\)

Signatures

Summary

pgr_edmondsKarp(Edges SQL, start vid, end vid)

pgr_edmondsKarp(Edges SQL, start vid, end vids)

pgr_edmondsKarp(Edges SQL, start vids, end vid)

pgr_edmondsKarp(Edges SQL, start vids, end vids)

pgr_edmondsKarp(Edges SQL, Combinations SQL)

Returns set of (seq, edge, start_vid, end_vid, flow, residual_capacity)

OR EMPTY SET

One to One

pgr_edmondsKarp(Edges SQL, start vid, end vid)

Returns set of (seq, edge, start_vid, end_vid, flow, residual_capacity)

OR EMPTY SET

Example:

From vertex \(11\) to vertex \(12\)

SELECT * FROM pgr_edmondsKarp(
  'SELECT id, source, target, capacity, reverse_capacity
  FROM edges',
  11, 12);
 seq | edge | start_vid | end_vid | flow | residual_capacity
-----+------+-----------+---------+------+-------------------
   1 |   10 |         7 |       8 |  100 |                30
   2 |   12 |         8 |      12 |  100 |                 0
   3 |    8 |        11 |       7 |  100 |                30
   4 |   11 |        11 |      12 |  130 |                 0
(4 rows)

One to Many

pgr_edmondsKarp(Edges SQL, start vid, end vids)

Returns set of (seq, edge, start_vid, end_vid, flow, residual_capacity)

OR EMPTY SET

Example:

From vertex \(11\) to vertices \(\{5, 10, 12\}\)

SELECT * FROM pgr_edmondsKarp(
  'SELECT id, source, target, capacity, reverse_capacity
  FROM edges',
  11, ARRAY[5, 10, 12]);
 seq | edge | start_vid | end_vid | flow | residual_capacity
-----+------+-----------+---------+------+-------------------
   1 |    1 |         6 |       5 |   50 |                80
   2 |    4 |         7 |       6 |   50 |                 0
   3 |   10 |         7 |       8 |   80 |                50
   4 |   12 |         8 |      12 |   80 |                20
   5 |    8 |        11 |       7 |  130 |                 0
   6 |   11 |        11 |      12 |  130 |                 0
   7 |    9 |        11 |      16 |   80 |                50
   8 |    3 |        15 |      10 |   80 |                50
   9 |   16 |        16 |      15 |   80 |                 0
(9 rows)

Many to One

pgr_edmondsKarp(Edges SQL, start vids, end vid)

Returns set of (seq, edge, start_vid, end_vid, flow, residual_capacity)

OR EMPTY SET

Example:

From vertices \(\{11, 3, 17\}\) to vertex \(12\)

SELECT * FROM pgr_edmondsKarp(
  'SELECT id, source, target, capacity, reverse_capacity
  FROM edges',
  ARRAY[11, 3, 17], 12);
 seq | edge | start_vid | end_vid | flow | residual_capacity
-----+------+-----------+---------+------+-------------------
   1 |    7 |         3 |       7 |   50 |                 0
   2 |   10 |         7 |       8 |  100 |                30
   3 |   12 |         8 |      12 |  100 |                 0
   4 |    8 |        11 |       7 |   50 |                80
   5 |   11 |        11 |      12 |  130 |                 0
(5 rows)

Many to Many

pgr_edmondsKarp(Edges SQL, start vids, end vids)

Returns set of (seq, edge, start_vid, end_vid, flow, residual_capacity)

OR EMPTY SET

Example:

From vertices \(\{11, 3, 17\}\) to vertices \(\{5, 10, 12\}\)

SELECT * FROM pgr_edmondsKarp(
  'SELECT id, source, target, capacity, reverse_capacity
  FROM edges',
  ARRAY[11, 3, 17], ARRAY[5, 10, 12]);
 seq | edge | start_vid | end_vid | flow | residual_capacity
-----+------+-----------+---------+------+-------------------
   1 |    7 |         3 |       7 |   50 |                 0
   2 |    1 |         6 |       5 |   50 |                80
   3 |    4 |         7 |       6 |   50 |                 0
   4 |   10 |         7 |       8 |  100 |                30
   5 |   12 |         8 |      12 |  100 |                 0
   6 |    8 |        11 |       7 |  100 |                30
   7 |   11 |        11 |      12 |  130 |                 0
   8 |    9 |        11 |      16 |   80 |                50
   9 |    3 |        15 |      10 |   80 |                50
  10 |   16 |        16 |      15 |   80 |                 0
(10 rows)

Combinations

pgr_edmondsKarp(Edges SQL, Combinations SQL)

Returns set of (seq, edge, start_vid, end_vid, flow, residual_capacity)

OR EMPTY SET

Example:

Using a combinations table, equivalent to calculating result from vertices \(\{5, 6\}\) to vertices \(\{10, 15, 14\}\).

The combinations table:

SELECT source, target FROM combinations
WHERE target NOT IN (5, 6);
 source | target
--------+--------
      5 |     10
      6 |     15
      6 |     14
(3 rows)

The query:

SELECT * FROM pgr_edmondsKarp(
  'SELECT id, source, target, capacity, reverse_capacity
  FROM edges',
  'SELECT * FROM combinations WHERE target NOT IN (5, 6)');
 seq | edge | start_vid | end_vid | flow | residual_capacity
-----+------+-----------+---------+------+-------------------
   1 |    4 |         6 |       7 |   80 |                20
   2 |    8 |         7 |      11 |   80 |                20
   3 |    9 |        11 |      16 |   80 |                50
   4 |   16 |        16 |      15 |   80 |                 0
(4 rows)

Parameters

Column	Type	Description
Edges SQL	TEXT	Edges SQL as described below
Combinations SQL	TEXT	Combinations SQL as described below
start vid	BIGINT	Identifier of the starting vertex of the path.
start vids	ARRAY[BIGINT]	Array of identifiers of starting vertices.
end vid	BIGINT	Identifier of the ending vertex of the path.
end vids	ARRAY[BIGINT]	Array of identifiers of ending vertices.

Inner Queries

Edges SQL

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.
capacity	ANY-INTEGER		Weight of the edge (source, target)
reverse_capacity	ANY-INTEGER	-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

Combinations SQL

Parameter	Type	Description
source	ANY-INTEGER	Identifier of the departure vertex.
target	ANY-INTEGER	Identifier of the arrival vertex.

Where:

ANY-INTEGER:

SMALLINT, INTEGER, BIGINT

Result columns

Column	Type	Description
seq	INT	Sequential value starting from 1.
edge	BIGINT	Identifier of the edge in the original query (edges_sql).
start_vid	BIGINT	Identifier of the first end point vertex of the edge.
end_vid	BIGINT	Identifier of the second end point vertex of the edge.
flow	BIGINT	Flow through the edge in the direction (start_vid, end_vid).
residual_capacity	BIGINT	Residual capacity of the edge in the direction (start_vid, end_vid).

Additional Examples

Example:

Manually assigned vertex combinations.

SELECT * FROM pgr_edmondsKarp(
  'SELECT id, source, target, capacity, reverse_capacity
  FROM edges',
  'SELECT * FROM (VALUES (5, 10), (6, 15), (6, 14)) AS t(source, target)');
 seq | edge | start_vid | end_vid | flow | residual_capacity
-----+------+-----------+---------+------+-------------------
   1 |    4 |         6 |       7 |   80 |                20
   2 |    8 |         7 |      11 |   80 |                20
   3 |    9 |        11 |      16 |   80 |                50
   4 |   16 |        16 |      15 |   80 |                 0
(4 rows)

See Also

• Flow - Family of functions

  • pgr_boykovKolmogorov

  • pgr_pushRelabel

• https://www.boost.org/libs/graph/doc/edmonds_karp_max_flow.html

• https://en.wikipedia.org/wiki/Edmonds%E2%80%93Karp_algorithm

Indices and tables

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