# pgr_connectedComponents¶

pgr_connectedComponents — Connected components of an undirected graph using a DFS-based approach.

Availability

• Version 3.0.0
• Return columns change:
• n_seq is removed
• seq changed type to BIGINT
• Official function
• Version 2.5.0
• New experimental function

Support

• Supported versions: current(3.1) 3.0
• Unsupported versions: 2.6 2.5

## Description¶

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

The main characteristics are:

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

## Signatures¶

pgr_connectedComponents(edges_sql)

RETURNS SET OF (seq, component, node)
OR EMPTY SET

Example: The connected components of the graph
SELECT * FROM pgr_connectedComponents(
'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 below.

## Inner query¶

edges SQL: an SQL query of an undirected 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 SMALLINT, INTEGER, BIGINT, REAL, FLOAT

## Result Columns¶

Returns set of (seq, component, node)

Column Type Description
seq BIGINT 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.