DISTINCTCOUNTTHETASKETCH
This section contains reference documentation for the DISTINCTCOUNTTHETASKETCH function.
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:4.2.0
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): Semicolon-separated parameter string for constructing the intermediate theta-sketches.The supported parameters are:
nominalEntries
: The nominal entries used to create the sketch. (Default 4096)samplingProbability
: Sets the upfront uniform sampling probability, p. (Default 1.0)accumulatorThreshold
: How many sketches should be kept in memory before merging. (Default 2)
predicates
(optional)_: _ These are individual predicates of formlhs <op> rhs
which are applied on rows selected by thewhere
clause. During intermediate sketch aggregation, sketches from thethetaSketchColumn
that satisfies these predicates are unionized individually. For example, all filtered rows that matchcountry=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 areSET_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.
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:
(the yearId 1986
is the only one in common)
By running the following query:
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