This is my favorite function in PgRouting . Here we will see an example. For more info, please see http://pgrouting.postlbs.org/wiki/ShootingStar

Let's suppose that we have the following network with two roundabouts as shown in the following picture. The SQL file used here is available at http://www.davidgis.fr/download/pgrouting_wr.sql


The network
The associated table is named wr. The wr's content is 
routing=# SELECT gid as id, source, target, cost, reverse_cost,x1, y1,x2, y2, rule, to_cost FROM wr order by id;
 id | source | target |       cost       |   reverse_cost   |        x1        |        y1         |        x2        |        y2         | rule | to_cost 
----+--------+--------+------------------+------------------+------------------+-------------------+------------------+-------------------+------+---------
  1 |      1 |      2 | 14.2572101668807 |             1000 | 76.7176891326003 |   33.775246780918 | 87.3962905382969 |  43.2217018705726 |      |        
  2 |      2 |      3 | 11.0893168443772 |             1000 | 87.3962905382969 |  43.2217018705726 | 87.3962905382969 |  54.3110187149498 |      |        
  3 |      3 |      4 | 13.9884637834871 |             1000 | 87.3962905382969 |  54.3110187149498 | 76.7176891326003 |  63.3467583659239 |      |        
  4 |      4 |      5 | 14.2572101668807 |             1000 | 76.7176891326003 |  63.3467583659239 | 67.2712340429456 |  52.6681569602273 |      |        
  5 |      5 |      6 | 10.6864968606796 |             1000 | 67.2712340429456 |  52.6681569602273 |  66.860518604265 |  41.9895555545307 |      |        
  6 |      6 |      1 | 12.8311604873812 |             1000 |  66.860518604265 |  41.9895555545307 | 76.7176891326003 |   33.775246780918 |      |        
  7 |      7 |      1 | 53.8753258085636 | 53.8753258085636 |               80 |               -20 | 76.7176891326003 |   33.775246780918 |      |        
  8 |      8 |      4 | 62.5380849791951 | 62.5380849791951 | 73.0212501844746 |  125.775505045381 | 76.7176891326003 |  63.3467583659239 |      |        
  9 |      7 |      9 |               40 |               40 |               80 |               -20 |              120 |               -20 |      |        
 10 |      9 |     10 | 33.4201752293529 | 33.4201752293529 |              120 |               -20 | 141.610728444141 | -45.4928328844451 |      |        
 11 |      9 |     11 |               60 |               60 |              120 |               -20 |              180 |               -20 |      |        
 12 |     11 |     12 | 25.9044654619518 | 25.9044654619518 |              180 |               -20 | 180.217979680121 | -45.9035483231257 |      |        
 13 |     12 |     10 | 38.6094358307609 | 38.6094358307609 | 180.217979680121 | -45.9035483231257 | 141.610728444141 | -45.4928328844451 |      |        
 14 |     13 |     12 | 29.5829181472758 | 29.5829181472758 | 181.039410557482 | -75.4750599081316 | 180.217979680121 | -45.9035483231257 |      |        
 15 |     13 |     14 | 39.4286821133412 | 39.4286821133412 | 181.039410557482 | -75.4750599081316 | 141.610728444141 | -75.4750599081316 |      |        
 16 |     14 |     10 | 29.9822270236865 | 29.9822270236865 | 141.610728444141 | -75.4750599081316 | 141.610728444141 | -45.4928328844451 |      |        
 17 |     15 |     13 | 39.2224758594282 | 39.2224758594282 |              220 |               -80 | 181.039410557482 | -75.4750599081316 |      |        
 18 |     15 |     16 | 38.0005743220152 |             1000 |              220 |               -80 | 251.682466010552 | -79.1714988562574 |      |        
 19 |     16 |     15 | 38.0539027874633 |             1000 | 251.682466010552 | -79.1714988562574 |              220 |               -80 |      |        
 20 |     17 |     16 | 48.3246366252935 | 48.3246366252935 |              300 |               -80 | 251.682466010552 | -79.1714988562574 |      |        
 21 |     11 |     18 |  115.27849852631 |  115.27849852631 |              180 |               -20 | 295.218302510699 | -23.7249146343713 |      |        
 22 |     18 |     17 | 56.4778705670458 | 56.4778705670458 | 295.218302510699 | -23.7249146343713 |              300 |               -80 |      |        
(22 rows)

I. Roundabouts

We have two roundabouts. First done with (1,2,3,4,5,6) and secund done with (18,19). So that is the reason why reverse_cost is enough expensive: reverse_cost=1000

In a first time, we want the path from edge=20 to edge=8. Using shooting_star, we have 
routing=# SELECT * FROM shortest_path_shooting_star('SELECT gid as id, source, target, cost, reverse_cost,x1, y1,x2, y2, rule, to_cost FROM wr order by id',20,8,true,true);
 vertex_id | edge_id |       cost       
-----------+---------+------------------
        31 |      20 | 48.3246366252935
        29 |      19 | 38.0539027874633
        27 |      17 | 39.2224758594282
        23 |      14 | 29.5829181472758
        21 |      13 | 38.6094358307609
        17 |      10 | 33.4201752293529
        15 |       9 |               40
        12 |       7 | 53.8753258085636
         1 |       1 | 14.2572101668807
         2 |       2 | 11.0893168443772
         4 |       3 | 13.9884637834871
         6 |       8 | 62.5380849791951
(12 rows)
We have

Shortest path using shooting star from edge=20 to 8

II. Turn restrictions

Let's suppose now that we have following restrictions:

  1. from edge from 17 to 14 ;
  2. from edge from 16 to 10;
You can' tgo from 17 to 14 and from 16 to 10. The restricted edges are shown on the following picture


Restricted edges: You can't go from 17 to 14 and from 16 to 10

So we have to do

routing=# update wr set to_cost=100,rule='17' where gid=14;
UPDATE 1
routing=# update wr set to_cost=100,rule='16' where gid=10;
So we have 
routing=# SELECT edge_id FROM shortest_path_shooting_star('SELECT gid as id, source, target, cost, reverse_cost,x1, y1,x2, y2, rule, to_cost FROM wr order by id',20,8,true,true);
 edge_id 
---------
      20
      19
      17
      15
      16
      13
      12
      11
       9
       7
       1
       2
       3
       8
(14 rows)
Beautifull...The final result is

Shortest path from edge=20 to 8