pgr_topologicalSort - Experimental

pgr_topologicalSort — Returns the linear ordering of the vertices(s) for weighted directed acyclic graphs(DAG). In particular, the topological sort algorithm implemented by Boost.Graph.

_images/boost-inside.jpeg

Boost Graph Inside

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 ANY-INTEGER and ANY-NUMERICAL
    • 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

Support

  • Supported versions: current(3.1) 3.0
  • TBD

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.

This implementation can only be used with a directed graph with no cycles i.e. directed acyclic graph.

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:For a directed graph
SELECT * FROM pgr_topologicalsort(
  'SELECT id,source,target,cost,reverse_cost FROM edge_table1'
);
 seq | sorted_v
-----+----------
   1 |        0
   2 |        1
   3 |        3
   4 |        2
(4 rows)

Parameters

Parameter Type Default Description
edges_sql TEXT   SQL query as described above.

Inner query

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)

  • 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, sorted_v)

Column Type Description
seq INT Sequential value starting from 1.
sorted_v BIGINT Linear ordering of the vertices(ordered in topological order)

See Also

Indices and tables