A metric aggregation that aggregates a set of points as
a GeoJSON LineString ordered by some sort parameter.
#### specifics
A `geo_line` aggregation request would specify a `geo_point` field, as well
as a `sort` field. `geo_point` represents the values used in the LineString,
while the `sort` values will be used as the total ordering of the points.
the `sort` field would support any numeric field, including date.
#### sample usage
```
{
"query": {
"bool": {
"must": [
{ "term": { "person": "004" } },
{ "term": { "trajectory": "20090131002206.plt" } }
]
}
},
"aggs": {
"make_line": {
"geo_line": {
"point": {"field": "location"},
"sort": { "field": "timestamp" },
"include_sort": true,
"sort_order": "desc",
"size": 15
}
}
}
}
```
#### sample response
```
{
"took": 21,
"timed_out": false,
"_shards": {...},
"hits": {...},
"aggregations": {
"make_line": {
"type": "LineString",
"coordinates": [
[
121.52926194481552,
38.92878997139633
],
[
121.52922699227929,
38.92876998055726
],
]
}
}
}
```
#### visual response
<img width="540" alt="Screen Shot 2019-04-26 at 9 40 07 AM" src="https://user-images.githubusercontent.com/388837/56834977-cf278e00-6827-11e9-9c93-005ed48433cc.png">
#### limitations
Due to the cardinality of points, an initial max of 10k points
will be used. This should support many use-cases.
One solution to overcome this limitation is to keep a PriorityQueue of
points, and simplifying the line once it hits this max. If simplifying
makes sense, it may be a nice option, in general. The ability to use a parameter
to specify how aggressive one wants to simplify. This parameter could be
the number of points. Example algorithm one could use with a PriorityQueue:
https://bost.ocks.org/mike/simplify/. This would still require O(m) space, where m
is the number of points returned. And would also require heapifying triangles
sorted by their areas, which would be O(log(m)) operations. Since sorting is done,
anyways, simplifying would still be a O(n log(m)) operation, where n is the total number
of points to filter........... something to explore
closes#41649
b514d9bf2e
