pgr_contractGraph
— Performs graph contraction and returns the contracted vertices and edges.
Availability: 2.3.0
Warning
Experimental functions
Contraction reduces the size of the graph by removing some of the vertices and edges and, for example, might add edges that represent a sequence of original edges decreasing the total time and space used in graph algorithms.
The pgr_contractGraph function has the following signatures:
pgr_contractGraph(edges_sql, contraction_order)
pgr_contractGraph(edges_sql, contraction_order, max_cycles, forbidden_vertices, directed)
RETURNS SETOF (seq, type, id, contracted_vertices, source, target, cost)
pgr_contractGraph(edges_sql, contraction_order)
Example: | Making a dead end contraction and a linear contraction. |
---|
SELECT * FROM pgr_contractGraph(
'SELECT id, source, target, cost, reverse_cost FROM edge_table',
ARRAY[1, 2]);
seq | type | id | contracted_vertices | source | target | cost
-----+------+----+---------------------+--------+--------+------
1 | v | 5 | {7,8} | -1 | -1 | -1
2 | v | 15 | {14} | -1 | -1 | -1
3 | v | 17 | {16} | -1 | -1 | -1
4 | e | -1 | {1,2} | 3 | 5 | 2
5 | e | -2 | {4} | 9 | 3 | 2
6 | e | -3 | {10,13} | 5 | 11 | 2
7 | e | -4 | {12} | 11 | 9 | 2
(7 rows)
pgr_contractGraph(edges_sql, contraction_order, max_cycles, forbidden_vertices, directed)
Example: | Making a dead end contraction and a linear contraction and vertex 2 is forbidden from contraction |
---|
SELECT * FROM pgr_contractGraph(
'SELECT id, source, target, cost, reverse_cost FROM edge_table',
ARRAY[1, 2], forbidden_vertices:=ARRAY[2]);
seq | type | id | contracted_vertices | source | target | cost
-----+------+----+---------------------+--------+--------+------
1 | v | 2 | {1} | -1 | -1 | -1
2 | v | 5 | {7,8} | -1 | -1 | -1
3 | v | 15 | {14} | -1 | -1 | -1
4 | v | 17 | {16} | -1 | -1 | -1
5 | e | -1 | {4} | 9 | 3 | 2
6 | e | -2 | {10,13} | 5 | 11 | 2
7 | e | -3 | {12} | 11 | 9 | 2
(7 rows)
edges_sql: | an SQL query, 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)
|
|
reverse_cost | ANY-NUMERICAL |
-1 | Weight of the edge (target, source),
|
Where:
ANY-INTEGER: | SMALLINT, INTEGER, BIGINT |
---|---|
ANY-NUMERICAL: | SMALLINT, INTEGER, BIGINT, REAL, FLOAT |
Column | Type | Description |
---|---|---|
edges_sql | TEXT |
SQL query as described above. |
contraction_order | ARRAY[ANY-INTEGER] |
|
forbidden_vertices | ARRAY[ANY-INTEGER] |
(optional). Identifiers of vertices forbidden from contraction. Default is an empty array. |
max_cycles | INTEGER |
(optional). Number of times the contraction operations on contraction_order will be performed. Default is 1. |
directed | BOOLEAN |
|
RETURNS SETOF (seq, type, id, contracted_vertices, source, target, cost)
The function returns a single row. The columns of the row are:
Column | Type | Description |
---|---|---|
seq | INTEGER |
Sequential value starting from 1. |
type | TEXT |
|
id | BIGINT |
|
contracted_vertices | ARRAY[BIGINT] |
Array of contracted vertex identifiers. |
source | BIGINT |
Identifier of the source vertex of the current edge id. Valid values when type = ‘e’. |
target | BIGINT |
Identifier of the target vertex of the current edge id. Valid values when type = ‘e’. |
cost | FLOAT |
Weight of the edge (source, target). Valid values when type = ‘e’. |
Example: | Only dead end contraction |
---|
SELECT * FROM pgr_contractGraph(
'SELECT id, source, target, cost, reverse_cost FROM edge_table',
ARRAY[1]);
seq | type | id | contracted_vertices | source | target | cost
-----+------+----+---------------------+--------+--------+------
1 | v | 2 | {1} | -1 | -1 | -1
2 | v | 5 | {7,8} | -1 | -1 | -1
3 | v | 10 | {13} | -1 | -1 | -1
4 | v | 15 | {14} | -1 | -1 | -1
5 | v | 17 | {16} | -1 | -1 | -1
(5 rows)
Example: | Only linear contraction |
---|
SELECT * FROM pgr_contractGraph(
'SELECT id, source, target, cost, reverse_cost FROM edge_table',
ARRAY[2]);
seq | type | id | contracted_vertices | source | target | cost
-----+------+----+---------------------+--------+--------+------
1 | e | -1 | {4} | 9 | 3 | 2
2 | e | -2 | {8} | 5 | 7 | 2
3 | e | -3 | {8} | 7 | 5 | 2
4 | e | -4 | {12} | 11 | 9 | 2
(4 rows)
Indices and tables