# 2. Pedestrian Routing¶

**pgRouting** was first called *pgDijkstra*, because it implemented only shortest
path search with *Dijkstra* algorithm. Later other functions were added and the
library was renamed to pgRouting.

Chapter Contents

## 2.1. pgr_dijkstra¶

Dijkstra algorithm was the first algorithm implemented in pgRouting. It doesn’t
require other attributes than `id`

, `source`

and `target`

ID and `cost`

and `reverse_cost`

.

You can specify when to consider the graph as directed or undirected.

Signature Summary

```
pgr_dijkstra(Edges SQL, start_vid, end_vid [, directed])
pgr_dijkstra(Edges SQL, start_vid, end_vids [, directed])
pgr_dijkstra(Edges SQL, start_vids, end_vid [, directed])
pgr_dijkstra(Edges SQL, start_vids, end_vids [, directed])
pgr_dijkstra(Edges SQL, Combinations SQL [, directed])
RETURNS SET OF (seq, path_seq [, start_vid] [, end_vid], node, edge, cost, agg_cost)
OR EMPTY SET
```

Description of the parameters can be found in pgr_dijkstra.

Note

Many pgRouting functions have

`sql::text`

as one of their arguments. While this may look confusing at first, it makes the functions very flexible as the user can pass a`SELECT`

statement as function argument as long as the returned result contains the required number of attributes and the correct attribute names.Most of pgRouting implemented algorithms do not require the

**geometry**.The pgRouting functions

**do not**return a geometry, but only an ordered list of nodes or edges.

Identifiers for the Queries

The assignment of the vertices identifiers on the source and target columns may be different, the following exercises will use the results of this query. For the workshop, some locations near of the FOSS4G Argentina event are going to be used. These locations are within this area https://www.openstreetmap.org#map=15/-34.5847/-58.3970

`192903446`

Plaza Intendente Alvear`4289340366`

Hard Rock Café`2153015792`

Facultad de Derecho`6357258588`

Centro de Convenciones Buenos Aires`196017392`

Palacio Duhau-Park Hyatt Buenos Aires

Connect to the database, if not connected:

```
psql city_routing
```

Get the vertex identifiers

```
1SELECT osm_id, id FROM ways_vertices_pgr
2WHERE osm_id IN (192903446, 4289340366, 2153015792, 6357258588, 196017392)
3ORDER BY osm_id;
```

```
1 osm_id | id
2------------+-------
3 192903446 | 1993
4 196017392 | 2197
5 2153015792 | 6646
6 4289340366 | 9129
7 6357258588 | 15011
8(5 rows)
9
```

`192903446`

Plaza Intendente Alvear (`1993`

)`4289340366`

Hard Rock Café (`9129`

)`2153015792`

Facultad de Derecho (`6646`

)`6357258588`

Centro de Convenciones Buenos Aires (`15011`

)`196017392`

Palacio Duhau-Park Hyatt Buenos Aires (`2197`

)

The corresponding `id`

are shown in the following image, and a sample route from
“Facultad de Derecho” to “Palacio Duhau-Park Hyatt Buenos Aires”.

### 2.1.1. Exercise 1: Single pedestrian routing¶

Problem:

Walking from “Plaza Intendente Alvear” to the “Facultad de Derecho”.

Solution:

The pedestrian wants to go from vertex

`1993`

to vertex`6646`

(lines**9**and**10**).The pedestrian’s cost is in terms of length. In this case

`length`

(line**6**), which was calculated by osm2pgrouting, is in unit`degrees`

.From a pedestrian perspective the graph is

`undirected`

(line**11**), that is, the pedestrian can move in both directions on all segments.

```
1SELECT * FROM pgr_dijkstra(
2 '
3 SELECT gid AS id,
4 source,
5 target,
6 length AS cost
7 FROM ways
8 ',
9 1993,
10 6646,
11 directed := false);
```

Exercise: 1 (Chapter: Pedestrian)

Note

The returned cost attribute represents the cost specified in the inner SQL query (

`edges_sql::text`

argument). In this example cost is`length`

in unit “degrees”. Cost may be time, distance or any combination of both or any other attributes or a custom formula.`node`

and`edge`

results may vary depending on the assignment of the identifiers to the vertices given by osm2pgrouting.

### 2.1.2. Exercise 2: Many Pedestrians going to the same destination¶

Problem:

Walking from the “Plaza Intendente Alvear” and “Hard Rock Café” to the “Facultad de Derecho”.

Solution:

The pedestrians are departing at vertices

`1993`

and`9129`

(line**9**).All pedestrians want to go to vertex

`6646`

(line**10**).The cost to be in meters using attribute

`length_m`

(line**6**).

```
1SELECT * FROM pgr_dijkstra(
2 '
3 SELECT gid AS id,
4 source,
5 target,
6 length_m AS cost
7 FROM ways
8 ',
9ARRAY[1993,9129],
106646,
11directed := false);
```

### 2.1.3. Exercise 3: Many Pedestrians departing from the same location¶

Problem:

Walking from the “Facultad de Derecho” to the “Plaza Intendente Alvear” and “Hard Rock Café” (in seconds).

Solution:

All pedestrians are departing from vertex

`6646`

(line**9**).Pedestrians want to go to locations

`1993`

and`9129`

(line**10**).The cost to be in seconds, with a walking speed

`s = 1.3 m/s`

and`t = d/s`

(line**6**).

```
1SELECT * FROM pgr_dijkstra(
2 '
3 SELECT gid AS id,
4 source,
5 target,
6 length_m / 1.3 AS cost
7 FROM ways
8 ',
96646,
10ARRAY[1993,9129],
11directed := false);
```

### 2.1.4. Exercise 4: Many Pedestrians going to different destinations¶

Problem:

Walking from the hotels to the “Centro de Convenciones Buenos Aires” and “Palacio Duhau-Park Hyatt Buenos Aires” (in minutes).

Solution:

The pedestrians depart from

`1993`

and`9129`

(line**9**).The pedestrians want to go to destinations

`15011`

and`2197`

(line**10**).The cost to be in minutes, with a walking speed

`s = 1.3 m/s`

and`t = d/s`

(line**6**).Result adds the costs per destination.

```
1SELECT * FROM pgr_dijkstra(
2 '
3 SELECT gid AS id,
4 source,
5 target,
6 length_m / 1.3 / 60 AS cost
7 FROM ways
8 ',
9ARRAY[1993, 9129],
10ARRAY[15011, 2197],
11directed := false);
```

Exercise: 4 (Chapter: Pedestrian)

Note

Inspecting the results, looking for totals (edge = -1):

Going to vertex

`15011`

:from

`1993`

takes 8.84.. minutes (seq = 35)from

`9129`

takes 5.84.. minutes (seq = 74)

Going to vertex

`2197`

:from

`1993`

takes 7.44.. minutes (seq = 7)from

`9129`

takes 12.06.. minutes (seq = 55)

## 2.2. pgr_dijkstraCost¶

When the main goal is to calculate the total cost, without “inspecting” the pgr_dijkstra results,
using `pgr_dijkstraCost`

returns a more compact result.

Signature Summary

```
pgr_dijkstraCost(edges_sql, start_vid, end_vid [, directed])
pgr_dijkstraCost(edges_sql, start_vid, end_vids [, directed])
pgr_dijkstraCost(edges_sql, start_vids, end_vid [, directed])
pgr_dijkstraCost(edges_sql, start_vids, end_vids [, directed])
pgr_dijkstraCost(edges_sql, combinations_sql [, directed])
RETURNS SET OF (start_vid, end_vid, agg_cost)
OR EMPTY SET
```

Description of the parameters can be found in pgr_dijkstraCost

### 2.2.1. Exercise 5: Many Pedestrians going to different destinations returning aggregate costs¶

Problem:

Walking from the hotels to the “Centro de Convenciones Buenos Aires” or “Palacio Duhau-Park Hyatt Buenos Aires” (get only the cost in minutes).

Solution:

The pedestrians depart from

`1993`

and`9129`

(line**10**).The pedestrians want to go to destinations

`15011`

and`2197`

(line**11**).The cost to be in minutes, with a walking speed

`s = 1.3 m/s`

and`t = d/s`

(line**7**).Result as aggregated costs.

```
1SELECT *
2FROM pgr_dijkstraCost(
3 '
4 SELECT gid AS id,
5 source,
6 target,
7 length_m / 1.3 / 60 AS cost
8 FROM ways
9 ',
10ARRAY[1993, 9129],
11ARRAY[15011, 2197],
12directed := false);
```

Exercise: 5 (Chapter: Pedestrian)

Compare with Exercise 4: Many Pedestrians going to different destinations ‘s note.

### 2.2.2. Exercise 6: Many Pedestrians going to different destinations summarizing the total costs per departure¶

Problem:

Walking from the hotels to the “Centro de Convenciones Buenos Aires” or “Palacio Duhau-Park Hyatt Buenos Aires” (summarize cost in minutes).

Solution:

The pedestrians depart from

`1993`

and`9129`

(line**10**).The pedestrians want to go to destinations

`15011`

and`2197`

(line**11**).The cost to be in minutes, with a walking speed s = 1.3 m/s and t = d/s (line

**7**).Result adds the costs per destination.

```
1SELECT start_vid, sum(agg_cost)
2FROM pgr_dijkstraCost(
3 '
4 SELECT gid AS id,
5 source,
6 target,
7 length_m / 1.3 / 60 AS cost
8 FROM ways
9 ',
10 ARRAY[1993, 9129],
11 ARRAY[15011, 2197],
12 directed := false)
13GROUP BY start_vid
14ORDER BY start_vid;
```

Exercise: 6 (Chapter: Pedestrian)

Note

An interpretation of the result can be: In general, it is faster to depart from the “Hard Rock Café” than from the “Plaza Intendente Alvear”.