# 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

## 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

ANY-NUMERICAL

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.