Loss of contracted vertex information in linear contraction.

[rohithsankepally]

Sample data is used to demonstrate this bug.

Expected Behavior

SELECT * FROM pgr_contractGraph(
    'SELECT id, source, target, cost FROM edge_table
    WHERE id = ANY(ARRAY[8, 9, 11, 13]) AND cost > 0 
    UNION
    SELECT id, target, source, reverse_cost FROM edge_table
    WHERE id = ANY(ARRAY[15, 16]) AND reverse_cost > 0',
    ARRAY[2]::integer[], 1, ARRAY[]::BIGINT[], true);
 seq | type | id | contracted_vertices | source | target | cost
-----+------+----+---------------------+--------+--------+------
   1 | e    | -5 | {6, 9}              |      5 |     4  |    3
   2 | e    | -6 | {6, 9, 11, 12}      |      5 |     4  |    5
(2 rows)

Actual Behavior

SELECT * FROM pgr_contractGraph(
    'SELECT id, source, target, cost FROM edge_table
    WHERE id = ANY(ARRAY[8, 9, 11, 13]) AND cost > 0 
    UNION
    SELECT id, target, source, reverse_cost FROM edge_table
    WHERE id = ANY(ARRAY[15, 16]) AND reverse_cost > 0',
    ARRAY[2]::integer[], 1, ARRAY[]::BIGINT[], true);
 seq | type | id | contracted_vertices | source | target | cost
-----+------+----+---------------------+--------+--------+------
   1 | e    | -1 | {11}                |      6 |     12 |    2
   2 | e    | -2 | {11,12}             |      6 |      9 |    3
   3 | e    | -3 | {6}                 |      5 |      9 |    2
   4 | e    | -4 | {6,9}               |      5 |      4 |    3
(4 rows)

It can be noticed that the shortcuts added from vertex 5 to vertex 4 do not store the contracted vertices 11 and 12.

[cvvergara]

I wonder if you can draw this graph, because even that it comes from the sample data, its not quite the same:

SELECT id, source, target, cost FROM edge_table
    WHERE id = ANY(ARRAY[8, 9, 11, 13]) AND cost > 0 
    UNION
    SELECT id, target, source, reverse_cost FROM edge_table
    WHERE id = ANY(ARRAY[15, 16]) AND reverse_cost > 0'

FYI: you have the link to the sample data from version 2.3 when we are on 2.5

[cvvergara]

Related to #1002