pgr_contraction
¶
pgr_contraction
— Performs graph contraction and returns the contracted
vertices and edges.
Availability
- Version 3.0.0
- Return columns change:
seq
is removed - Name change from
pgr_contractGraph
- Bug fixes
- Official function
- Return columns change:
- Version 2.3.0
- New experimental function
Description¶
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 main Characteristics are:
- Process is done only on edges with positive costs.
- Does not return the full contracted graph
- Only changes on the graph are returned
- Currnetly there are two types of contraction methods
- Dead End Contraction
- Linear Contraction
- The returned values include
- the added edges by linear contraction.
- the modified vertices by dead end contraction.
- The returned values are ordered as follows:
- column
id
ascending when type isv
- column
id
descending when type ise
- column
Signatures¶
Summary
The pgr_contraction function has the following signature:
pgr_contraction(Edges SQL, contraction order [, max_cycles] [, forbidden_vertices] [, directed]) RETURNS SETOF (type, id, contracted_vertices, source, target, cost)
Example: | Making a dead end and linear contraction in that order on an undirected graph. |
---|
SELECT * FROM pgr_contraction(
'SELECT id, source, target, cost, reverse_cost FROM edges',
ARRAY[1, 2], directed => false);
type | id | contracted_vertices | source | target | cost
------+----+---------------------+--------+--------+------
v | 4 | {2} | -1 | -1 | -1
v | 7 | {1,3} | -1 | -1 | -1
v | 14 | {13} | -1 | -1 | -1
e | -1 | {5,6} | 7 | 10 | 2
e | -2 | {8,9} | 7 | 12 | 2
e | -3 | {17} | 12 | 16 | 2
e | -4 | {15} | 10 | 16 | 2
(7 rows)
Parameters¶
Parameter | Type | Description |
---|---|---|
Edges SQL | TEXT |
Edges SQL as described below. |
contraction Order | ARRAY[ ANY-INTEGER ] |
Ordered contraction operations.
|
Optional Parameters¶
Column | Type | Default | Description |
---|---|---|---|
directed |
BOOLEAN |
true |
|
Contraction optional parameters¶
Column | Type | Default | Description |
---|---|---|---|
forbidden_vertices |
ARRAY[ ANY-INTEGER ] |
Empty | Identifiers of vertices forbidden for contraction. |
max_cycles |
INTEGER |
\(1\) | Number of times the contraction operations on contraction_order will
be performed. |
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. | |
cost |
ANY-NUMERICAL | Weight of the edge (source , target ) |
|
reverse_cost |
ANY-NUMERICAL | -1 | Weight of the edge (
|
Where:
ANY-INTEGER: | SMALLINT , INTEGER , BIGINT |
---|---|
ANY-NUMERICAL: | SMALLINT , INTEGER , BIGINT , REAL , FLOAT |
Result Columns¶
RETURNS SETOF (type, id, contracted_vertices, source, target, cost)
The function returns a single row. The columns of the row are:
Column | Type | Description |
---|---|---|
type |
TEXT |
Type of the
|
id |
BIGINT |
All numbers on this column are
|
contracted_vertices |
ARRAY[BIGINT] |
Array of contracted vertex identifiers. |
source |
BIGINT |
|
target |
BIGINT |
|
cost |
FLOAT |
|
Additional Examples¶
Example: | Only dead end contraction |
---|
SELECT type, id, contracted_vertices FROM pgr_contraction(
'SELECT id, source, target, cost, reverse_cost FROM edges',
ARRAY[1]);
type | id | contracted_vertices
------+----+---------------------
v | 4 | {2}
v | 6 | {5}
v | 7 | {1,3}
v | 8 | {9}
v | 14 | {13}
(5 rows)
Example: | Only linear contraction |
---|
SELECT * FROM pgr_contraction(
'SELECT id, source, target, cost, reverse_cost FROM edges',
ARRAY[2]);
type | id | contracted_vertices | source | target | cost
------+----+---------------------+--------+--------+------
e | -1 | {3} | 1 | 7 | 2
e | -2 | {3} | 7 | 1 | 2
(2 rows)