Combining Graph Traversals

Finding the start vertex via a geo query

Our first example will locate the start vertex for a graph traversal via a geo index. We use the city graph and its geo indexes:

```arangosh> var examples = require("@arangodb/graph-examples/example-graph.js");
arangosh> var g = examples.loadGraph("routeplanner");```

We search all german cities in a range of 400 km around the ex-capital Bonn: Hamburg and Cologne. We won’t find Paris since its in the `frenchCity` collection.

```FOR startCity IN germanCity
FILTER GEO_DISTANCE(@bonn, startCity.geometry) < @radius
RETURN startCity._key```
Bind Parameters:
```{
"bonn": [
7.0998,
50.734
],
"radius": 400000
}```
Result:
```[
"Cologne",
"Hamburg"
]```

Let’s revalidate that the geo indexes are actually used:

```FOR startCity IN germanCity
FILTER GEO_DISTANCE(@bonn, startCity.geometry) < @radius
RETURN startCity._key```
Bind Parameters:
```{
"bonn": [
7.0998,
50.734
],
"radius": 400000
}```
Explain:
```Query String (119 chars, cacheable: true):
FOR startCity IN germanCity
FILTER GEO_DISTANCE(@bonn, startCity.geometry) < @radius
RETURN startCity._key

Execution plan:
Id   NodeType          Est.   Comment
1   SingletonNode        1   * ROOT
7   IndexNode            0     - FOR startCity IN germanCity   /* geo index scan, index scan + document lookup (projections: `_key`) */
5   CalculationNode      0       - LET #3 = startCity.`_key`   /* attribute expression */   /* collections used: startCity : germanCity */
6   ReturnNode           0       - RETURN #3

Indexes used:
By   Name                      Type   Collection   Unique   Sparse   Cache   Selectivity   Fields           Stored values   Ranges
7   idx_1761065949175218177   geo    germanCity   false    true     false           n/a   [ `geometry` ]   [  ]            (GEO_DISTANCE([ 7.0998, 50.734 ], startCity.`geometry`) < 400000)

Optimization rules applied:
Id   RuleName
1   move-calculations-up
2   move-filters-up
3   move-calculations-up-2
4   move-filters-up-2
5   geo-index-optimizer
6   remove-unnecessary-calculations-2
7   reduce-extraction-to-projection

44 rule(s) executed, 1 plan(s) created, peak mem [b]: 0, exec time [s]: 0.00021```

And now combine this with a graph traversal:

```FOR startCity IN germanCity
FILTER GEO_DISTANCE(@bonn, startCity.geometry) < @radius
FOR v, e, p IN 1..1 OUTBOUND startCity
GRAPH 'routeplanner'
RETURN {startcity: startCity._key, traversedCity: v._key}```
Bind Parameters:
```{
"bonn": [
7.0998,
50.734
],
"radius": 400000
}```
Result:
```[
{
"startcity": "Cologne",
"traversedCity": "Paris"
},
{
"startcity": "Cologne",
"traversedCity": "Lyon"
},
{
"startcity": "Hamburg",
"traversedCity": "Lyon"
},
{
"startcity": "Hamburg",
"traversedCity": "Paris"
},
{
"startcity": "Hamburg",
"traversedCity": "Cologne"
}
]```

The geo index query returns us `startCity` (Cologne and Hamburg) which we then use as starting point for our graph traversal. For simplicity we only return their direct neighbours. We format the return result so we can see from which `startCity` the traversal came.

Alternatively we could use a `LET` statement with a subquery to group the traversals by their `startCity` efficiently:

```FOR startCity IN germanCity
FILTER GEO_DISTANCE(@bonn, startCity.geometry) < @radius
LET oneCity = (
FOR v, e, p IN 1..1 OUTBOUND startCity
GRAPH 'routeplanner' RETURN v._key
)
RETURN {startCity: startCity._key, connectedCities: oneCity}```
Bind Parameters:
```{
"bonn": [
7.0998,
50.734
],
"radius": 400000
}```
Result:
```[
{
"startCity": "Cologne",
"connectedCities": [
"Paris",
"Lyon"
]
},
{
"startCity": "Hamburg",
"connectedCities": [
"Lyon",
"Paris",
"Cologne"
]
}
]```

Finally, we clean up again:

`arangosh> examples.dropGraph("routeplanner");`