When "directed" option is enabled, "No paths found in (U) pgr_..." error is shown

[sanak]

Problem
From the following issue's comment:
#133 (comment)

:
However none of the functions work with directed option enabled (pgrouting 3.3.0 , osm2pgrouting 2.3.8.1) Sql querys work

No paths found in (U) ...

Here is the error message when using pgr_dijkstra.

To Reproduce

1. Check on "directed" option
2. Execute "Preview" function

Expectation
The error message should not shown, and the directed result should be drawn.

[sanak]

@jonath35 On my environment, turning on Reverse Cost and set appropriate column name solved the above No paths found in (U) pgr_... issue when directed is enabled.

If you don't set Reverse Cost, could you try above once ?

[cvvergara]

Ah, interesting ...

no reverse_cost

For this structure edge_table1 without reverse_cost

gid	source	target	cost
1000	1	2	23
1001	2	1	23

Without the tick mark:
SELECT gid AS id, source, target, cost FROM edge_table1
This represents,1<--->2
With the tick mark:
There would be an error because the reverse_cost column does not exist on the edge_table1

with reverse_cost

For this structure edge_table2 with reverse_cost

id	source	target	cost	reverse cost
1000	1	2	23	23

Without the tick mark:
These represents,1<--->2
SELECT gid AS id, source, target, cost, reverse_cost FROM edge_table2
This is weird but also works
SELECT gid AS id, source, target, reverse_cost AS cost, cost AS reverse_cost FROM edge_table2

Without the tick mark:
But if the tick is not there then the query of the image above is
SELECT gid AS id, source, target, cost FROM edge_table2
This represents,1--->2
path from 2 to 1 will not be found

If the word reverse_cost is on the Cost field the query would be
SELECT gid AS id, source, target, reverse_cost AS cost FROM edge_table2
This represents,1<---2
path from 1 to 2 will not be found
In these last 2 cases, the graph is not complete as you want 1<-->2 as the graph and no path will be found depending on the query.

conclusion

• Have a tick mark on the Reverse_cost field as default as the data from osm2pgrouting has the reverse_cost column
• Send an appropiate message when the reverse_cost column does not exist on the inner query
• Beautify the message:
  
  For example

No paths found in pgr_dijkstra:
10605
to 
11481
* Does the query need `reverse_cost` column?
* Are both vertices in the graph?
* Are both vertices in the same graph component?
* Are both vertices in the same graph strong component?

[sanak]

@cvvergara Okay, thanks for the details!

About graph related part, I may not understand correctly, so I will try to check those in the following example data, later.
https://docs.pgrouting.org/3.3/en/sampledata.html

About the conclusion part,

• Have a tick mark on the Reverse_cost field as default as the data from osm2pgrouting has the reverse_cost column

Okay, sure. Default tick mark=on will have less troubles.

• Send an appropiate message when the reverse_cost column does not exist on the inner query

Okay, but is it better to show (raise) this message before running pgr_xxx query in pgRoutingLayer side ?
Or is there possibility to show error in each pgr_xxx functions initial validation process ?