This module provides Apache Druid aggregators based on Tuple sketch from [Apache DataSketches](https://datasketches.apache.org/) library. ArrayOfDoublesSketch sketches extend the functionality of the count-distinct Theta sketches by adding arrays of double values associated with unique keys.
|nominalEntries|Parameter that determines the accuracy and size of the sketch. Higher k means higher accuracy but more space to store sketches. Must be a power of 2. See the [Theta sketch accuracy](https://datasketches.apache.org/docs/Theta/ThetaErrorTable) for details. |no, defaults to 16384|
|metricColumns|When building sketches from raw data, an array input column that contain numeric values to associate with each distinct key. If not provided, assumes `fieldName` is an `arrayOfDoublesSketch`|no, if not provided `fieldName` is assumed to be an arrayOfDoublesSketch|
|numberOfValues|Number of values associated with each distinct key. |no, defaults to the length of `metricColumns` if provided and 1 otherwise|
You can use the `arrayOfDoublesSketch` aggregator to:
- Build a sketch from raw data. In this case, set `metricColumns` to an array.
- Build a sketch from an existing `ArrayOfDoubles` sketch . In this case, leave `metricColumns` unset and set the `fieldName` to an `ArrayOfDoubles` sketch with `numberOfValues` doubles. At ingestion time, you must base64 encode `ArrayOfDoubles` sketches at ingestion time.
#### Example on top of raw data
Compute a theta of unique users. For each user store the `added` and `deleted` scores. The new sketch column will be called `users_theta`.
```json
{
"type": "arrayOfDoublesSketch",
"name": "users_theta",
"fieldName": "user",
"nominalEntries": 16384,
"metricColumns": ["added", "deleted"],
}
```
#### Example ingesting a precomputed sketch column
Ingest a sketch column called `user_sketches` that has a base64 encoded value of two doubles in its array and store it in a column called `users_theta`.
#### Estimate of the number of distinct keys with error bounds
Returns a distinct count estimate and error bounds from a given ArrayOfDoublesSketch. The result will be three double values: estimate of the number of distinct keys, lower bound and upper bound. The bounds are provided at the given number of standard deviations (optional, defaults to 1). This must be an integer value of 1, 2 or 3 corresponding to approximately 68.3%, 95.4% and 99.7% confidence intervals.
Returns a list of mean values from a given ArrayOfDoublesSketch. The result will be N double values, where N is the number of double values kept in the sketch per key.
Returns a list of variance values from a given ArrayOfDoublesSketch. The result will be N double values, where N is the number of double values kept in the sketch per key.
Returns a quantiles DoublesSketch constructed from a given column of values from a given ArrayOfDoublesSketch using optional parameter k that determines the accuracy and size of the quantiles sketch. See [Quantiles Sketch Module](datasketches-quantiles.md)
Returns a result of a specified set operation on the given array of sketches. Supported operations are: union, intersection and set difference (UNION, INTERSECT, NOT).
Performs Student's t-test and returns a list of p-values given two instances of ArrayOfDoublesSketch. The result will be N double values, where N is the number of double values kept in the sketch per key. See [t-test documentation](https://commons.apache.org/proper/commons-math/javadocs/api-3.6.1/org/apache/commons/math3/stat/inference/TTest.html).
Returns a human-readable summary of a given ArrayOfDoublesSketch. This is a string returned by toString() method of the sketch. This can be useful for debugging.
