DISTINCTCOUNTTHETASKETCH

The Theta Sketch framework enables set operations over a stream of data, and can also be used for cardinality estimation. Pinot leverages the Sketch Class and its extensions from the library org.apache.datasketches:datasketches-java:1.2.0-incubating to perform distinct counting as well as evaluating set operations.

Signature

DistinctCountThetaSketch(<thetaSketchColumn>, <thetaSketchParams>, predicate1, predicate2..., postAggregationExpressionToEvaluate) -> Long

• thetaSketchColumn (required): Name of the column to aggregate on.

• thetaSketchParams (required): Parameters for constructing the intermediate theta-sketches.

  • Currently, the only supported parameter is nominalEntries (defaults to 4096).

• predicates (optional)_: _ These are individual predicates of form lhs <op> rhs which are applied on rows selected by the where clause. During intermediate sketch aggregation, sketches from the thetaSketchColumn that satisfies these predicates are unionized individually. For example, all filtered rows that match country=USA are unionized into a single sketch. Complex predicates that are created by combining (AND/OR) of individual predicates is supported.

• postAggregationExpressionToEvaluate (required): The set operation to perform on the individual intermediate sketches for each of the predicates. Currently supported operations are SET_DIFF, SET_UNION, SET_INTERSECT , where DIFF requires two arguments and the UNION/INTERSECT allow more than two arguments.

Usage Examples

These examples are based on the Batch Quick Start.

select distinctCountThetaSketch(teamID) AS value
from baseballStats 

select distinctCountThetaSketch(teamID, 'nominalEntries=10') AS value
from baseballStats

We can also provide predicates and a post aggregation expression to compute more complicated cardinalities. For example, we could can find the intersection of the following queries:

select yearID
from baseballStats
where teamID = 'SFN' AND numberOfGames = 28 AND homeRuns = 1

select yearID
from baseballStats
where teamID = 'CHN' AND numberOfGames = 28 AND homeRuns = 1

(the yearId 1986 is the only one in common)

By running the following query:

select distinctCountThetaSketch(
  yearID, 
  'nominalEntries=4096', 
  'teamID = ''SFN'' AND numberOfGames=28 AND homeRuns=1',
  'teamID = ''CHN'' AND numberOfGames=28 AND homeRuns=1',
  'SET_INTERSECT($1, $2)'
) AS value
from baseballStats