pgr_strongComponents

pgr_strongComponents — Strongly connected components of a directed graph using Tarjan’s algorithm based on DFS.

_images/boost-inside.jpeg

Boost Graph Inside

Availability

  • Version 3.0.0
    • Return columns change: n_seq is removed
    • Official function
  • Version 2.5.0
    • New experimental function

Support

  • Supported versions: current(3.0)
  • Unsupported versions: 2.6 2.5

Description

A strongly connected component of a directed graph is a set of vertices that are all reachable from each other.

The main characteristics are:

  • The signature is for a directed graph.
  • Components are described by vertices
  • The returned values are ordered:
    • component ascending
    • node ascending
  • Running time: \(O(V + E)\)

Signatures

pgr_strongComponents(Edges SQL)

RETURNS SET OF (seq, component, node)
OR EMPTY SET
Example:The strong components of the graph
SELECT * FROM pgr_strongComponents(
    'SELECT id, source, target, cost, reverse_cost FROM edge_table'
);
 seq | component | node
-----+-----------+------
   1 |         1 |    1
   2 |         1 |    2
   3 |         1 |    3
   4 |         1 |    4
   5 |         1 |    5
   6 |         1 |    6
   7 |         1 |    7
   8 |         1 |    8
   9 |         1 |    9
  10 |         1 |   10
  11 |         1 |   11
  12 |         1 |   12
  13 |         1 |   13
  14 |        14 |   14
  15 |        14 |   15
  16 |        16 |   16
  17 |        16 |   17
(17 rows)

Parameters

Parameter Type Default Description
Edges SQL TEXT   Inner query as described bellow.

Inner query

edges SQL:an SQL query of a directed graph, which should return a set of rows with the following columns:
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 (seq, component, node)

Column Type Description
seq INT Sequential value starting from 1.
component BIGINT Component identifier. It is equal to the minimum node identifier in the component.
node BIGINT Identifier of the vertex that belongs to component.

See Also

Indices and tables