pgr_topologicalSort
 Experimental¶
pgr_topologicalSort
— Linear ordering of the vertices for directed acyclic
graphs (DAG).
Warning
Possible server crash
These functions might create a server crash
Warning
Experimental functions
They are not officially of the current release.
They likely will not be officially be part of the next release:
The functions might not make use of ANYINTEGER and ANYNUMERICAL
Name might change.
Signature might change.
Functionality might change.
pgTap tests might be missing.
Might need c/c++ coding.
May lack documentation.
Documentation if any might need to be rewritten.
Documentation examples might need to be automatically generated.
Might need a lot of feedback from the comunity.
Might depend on a proposed function of pgRouting
Might depend on a deprecated function of pgRouting
Availability
Version 3.0.0
New experimental function
Description¶
The topological sort algorithm creates a linear ordering of the vertices such that if edge \((u,v)\) appears in the graph, then \(v\) comes before \(u\) in the ordering.
The main characteristics are:
Process is valid for directed acyclic graphs only. otherwise it will throw warnings.
 For optimization purposes, if there are more than one answer, the function
will return one of them.
The returned values are ordered in topological order:
Running time: \(O(V + E)\)
Signatures¶
Summary
pgr_topologicalSort(Edges SQL) RETURNS SET OF (seq, sorted_v) OR EMPTY SET
 Example:
Topologically sorting the graph
SELECT * FROM pgr_topologicalsort(
$$SELECT id, source, target, cost
FROM edges WHERE cost >= 0
UNION
SELECT id, target, source, reverse_cost
FROM edges WHERE cost < 0$$);
seq  sorted_v
+
1  1
2  5
3  2
4  4
5  3
6  13
7  14
8  15
9  10
10  6
11  7
12  8
13  9
14  11
15  16
16  12
17  17
(17 rows)
Parameters¶
Parameter 
Type 
Description 


Edges SQL as described below. 
Inner Queries¶
Edges SQL¶
Column 
Type 
Default 
Description 


ANYINTEGER 
Identifier of the edge. 


ANYINTEGER 
Identifier of the first end point vertex of the edge. 


ANYINTEGER 
Identifier of the second end point vertex of the edge. 


ANYNUMERICAL 
Weight of the edge ( 


ANYNUMERICAL 
1 
Weight of the edge (

Where:
 ANYINTEGER:
SMALLINT
,INTEGER
,BIGINT
 ANYNUMERICAL:
SMALLINT
,INTEGER
,BIGINT
,REAL
,FLOAT
Result Columns¶
Returns set of (seq, sorted_v)
Column 
Type 
Description 



Sequential value starting from \(1\) 


Linear topological ordering of the vertices 
Additional examples¶
 Example:
Topologically sorting the one way segments
SELECT * FROM pgr_topologicalsort(
$$SELECT id, source, target, cost, 1 AS reverse_cost
FROM edges WHERE cost >= 0
UNION
SELECT id, source, target, 1, reverse_cost
FROM edges WHERE cost < 0$$);
seq  sorted_v
+
1  5
2  2
3  4
4  13
5  14
6  1
7  3
8  15
9  10
10  6
11  7
12  8
13  9
14  11
15  12
16  16
17  17
(17 rows)
 Example:
Graph is not a DAG
SELECT * FROM pgr_topologicalsort(
$$SELECT id, source, target, cost, reverse_cost FROM edges$$);
ERROR: The graph must be a DAG.
HINT: Working with Directed Graph
CONTEXT: SQL function "pgr_topologicalsort" statement 1
See Also¶
Indices and tables