Druid - descending timestamps with groupBy query - database

Question

What I'm asking for should be very simple but the Druid docs have little to no info about this.
I am making a groupBy query, and the data is very large so I'm "paging" it by increasing limitSpec.limit on each subsequent query.
By default, the returned array starts from the beginning timestamp and moves forward in time. I want the results to start from the end timestamp and move backwards in time from there.
Does anyone know how to do that?
So in other words, by default a groupBy query would look like this:
[
{
"version" : "v1",
"timestamp" : "2012-01-01T00:00:00.000Z",
"event" : {
"total_usage" : <some_value_one>
}
},
{
"version" : "v1",
"timestamp" : "2012-01-02T00:00:00.000Z",
"event" : {
"total_usage" : <some_value_two>
}
}
]
Whereas I want it to look like this:
[
{
"version" : "v1",
"timestamp" : "2012-01-02T00:00:00.000Z",
"event" : {
"total_usage" : <some_value_two>
}
},
{
"version" : "v1",
"timestamp" : "2012-01-01T00:00:00.000Z",
"event" : {
"total_usage" : <some_value_one>
}
}
]

Answer 1

You can achieve the ordering by using the "columns" attribute in the limit spec. see the below example.
{
"type" : "default",
"limit" : <integer_value>,
"columns" : [list of OrderByColumnSpec],
}
For more details you can refer the below druid doc -
http://druid.io/docs/latest/querying/limitspec.html

Answer 2

You can add timestamp as a dimension but truncated to date (assuming you use day granularity in your query) and force Druid to sort the result first by dimension values and then by timestamp.
Example Query:
{
"dataSource": "your_datasource",
"queryType": "groupBy",
"dimensions": [
{
"type": "default",
"dimension": "some_dimension_in",
"outputName": "some_dimension_out",
"outputType": "STRING"
},
{
"type": "extraction",
"dimension": "__time",
"outputName": "__timestamp",
"extractionFn": {
"type": "timeFormat",
"format" : "yyyy-MM-dd"
}
}
],
"aggregations": [
{
"type": "doubleSum",
"name": "some_metric",
"fieldName": "some_metric_field"
}
],
"limitSpec": {
"type": "default",
"limit": 1000,
"columns": [
{
"dimension": "__timestamp",
"direction": "descending",
"dimensionOrder": "numeric"
},
{
"dimension": "some_metric",
"direction": "descending",
"dimensionOrder": "numeric"
}
]
},
"intervals": [
"2019-09-01/2019-10-01"
],
"granularity": "day",
"context": {
"sortByDimsFirst": "true"
}
}