How to get Quantile/median values in pydruid - druid

Question

My goal is to query the median value of column height in my druid datasource. I was able to use other aggregations like count and count distinct values. Here's my query so far:
group = query.groupby(
datasource=datasource,
granularity='all',
intervals='2020-01-01T00:00:00+00:00/2101-01-01T00:00:00+00:00',
dimensions=[
"category_a"
],
filter=(Dimension("country") == country_id),
aggregations={
'count': longsum('count'),
'count_distinct_city': aggregators.thetasketch('city'),
}
)
There's a class Quantile under postaggregator.py so I tried using this.
class Quantile(Postaggregator):
def __init__(self, name, probability):
Postaggregator.__init__(self, None, None, name)
self.post_aggregator = {
"type": "quantile",
"fieldName": name,
"probability": probability,
}
Here's my attempt at getting the median:
post_aggregations={
'median_value': postaggregator.Quantile(
'height', 50
)
}
The error I'm getting here is 'Could not resolve type id \'quantile\' as a subtype of [simple type, class io.druid.query.aggregation.PostAggregator]:
Druid Error: {'error': 'Unknown exception', 'errorMessage': 'Could not resolve type id \'quantile\' as a subtype of [simple type, class io.druid.query.aggregation.PostAggregator]: known type ids = [arithmetic, constant, doubleGreatest, doubleLeast, expression, fieldAccess, finalizingFieldAccess, hyperUniqueCardinality, javascript, longGreatest, longLeast, quantilesDoublesSketchToHistogram, quantilesDoublesSketchToQuantile, quantilesDoublesSketchToQuantiles, quantilesDoublesSketchToString, sketchEstimate, sketchSetOper, thetaSketchEstimate, thetaSketchSetOp] (for POJO property \'postAggregations\')\n at [Source: (org.eclipse.jetty.server.HttpInputOverHTTP); line: 1, column: 856] (through reference chain: io.druid.query.groupby.GroupByQuery["postAggregations"]->java.util.ArrayList[0])', 'errorClass': 'com.fasterxml.jackson.databind.exc.InvalidTypeIdException', 'host': None}

Answer 1

I modified the code of pydruid to get this working on our end. I've created new aggregator and postaggregator under /pydruid/utils.
aggregator.py
def quantilesDoublesSketch(raw_column, k=128):
return {"type": "quantilesDoublesSketch", "fieldName": raw_column, "k": k}
postaggregator.py
class QuantilesDoublesSketchToQuantile(Postaggregator):
def __init__(self, name: str, field_name: str, fraction: float):
self.post_aggregator = {
"type": "quantilesDoublesSketchToQuantile",
"name": name,
"fraction": fraction,
"field": {
"fieldName": field_name,
"name": field_name,
"type": "fieldAccess",
},
}
My first time to create a PR! Hopefully they accept and publish officially.
https://github.com/druid-io/pydruid/pull/287