Elastic get documents with min/max and last value - elasticsearch

Question

Let's say I have an elastic index with the following data:
{"var1": 14, "time": "2019-02-12T13:01:00.000Z"}
{"var2": 1423, "time": "2019-02-12T13:01:02.000Z"}
{"var3": 114, "time": "2019-02-12T13:01:03.000Z"}
{"var2": 214, "time": "2019-02-12T13:01:04.000Z"}
{"var3": 414, "time": "2019-02-12T13:01:05.000Z"}
{"var1": 124, "time": "2019-02-12T13:01:06.000Z"}
{"var2": 914, "time": "2019-02-12T13:01:07.000Z"}
{"var3": 8614, "time": "2019-02-12T13:01:06.000Z"}
{"var2": 74, "time": "2019-02-12T13:01:07.000Z"}
{"var3": 174, "time": "2019-02-12T13:01:08.000Z"}
{"var4": 144, "time": "2019-02-12T13:01:09.000Z"}
{"var4": 714, "time": "2019-02-12T13:01:10.000Z"}
{"var4": 813, "time": "2019-02-12T13:01:11.000Z"}
{"var2": 65, "time": "2019-02-12T13:01:12.000Z"}
{"var1": 321, "time": "2019-02-12T13:01:13.000Z"}
I would like to write ONE query that can retrieve the minimum of a variable, the maximum of a variable and the last n values of a variable in a given time interval.
It is important that I need the actual document that has the min or the max or the last value (this is why I'm using top_hits for the min and max instead of the min or max aggregations).
So far I have this query:
{
"query": {
"bool": {
"must": [
{
"range": {
"time": {
"gte": "2019-02-12T13:01:00.000Z",
"lt": "2019-02-12T13:01:15.000Z"
}
}
}
]
}
},
"size": 0,
"aggs": {
"max_var1": {
"top_hits": {
"size": 1,
"sort": [{
"var1": {"order": "desc"}
}]
}
},
"min_var2": {
"top_hits": {
"size": 1,
"sort": [{
"var2": {"order": "asc"}
}]
}
},
"last_var4": {
"top_hits": {
"size": 3,
"sort": [{
"time": {"order": "desc"}
}],
"_source": ["var4"]
}
}
}
}
The query returns correctly the min and the max value but it doesn't return the correct last 3 value for var4, because it takes the last from all the documents in the given time interval, and not the documents that have var4 in it.
So the question is how to get the last n documents for a given variable inside this query.
I know I could use the multi search API to execute several queries at once, but I would like to know if it is possible to have it in one query.
Thanks.

Answer 1

Filtered aggregation to the rescue. Simply make sure to constrain the last_var4 aggregation to only those docs that actually have the field var4.
{
...
"last_var4": {
"filter": {
"bool": {
"filter": {
"exists": {
"field": "var4"
}
}
}
},
"aggs": {
"last_var4": {
"top_hits": {
"size": 3,
"sort": [
{
"time": {
"order": "desc"
}
}
],
"_source": [
"var4"
]
}
}
}
}
}
}