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groupArrayResample

Description​

The Resample combinator can be applied to the groupArray aggregate function to divide the range of a specified key column into a fixed number of intervals (N) and construct the resulting array by selecting one representative value (corresponding to the minimum key) from the data points falling into each interval. It creates a downsampled view of the data rather than collecting all values.

Example usage​

Let's look at an example. We'll create a table which contains the name, age and wage of employees, and we'll insert some data into it:

CREATE TABLE employee_data 
(
    name String,
    age UInt8,
    wage Float32
) ENGINE = MergeTree()
ORDER BY tuple()

INSERT INTO employee_data (name, age, wage) VALUES
    ('John', 16, 10.0),
    ('Alice', 30, 15.0),
    ('Mary', 35, 8.0),
    ('Evelyn', 48, 11.5),
    ('David', 62, 9.9),
    ('Brian', 60, 16.0);

Let's get the names of the people whose age lies in the intervals of [30,60) and [60,75). Since we use integer representation for age, we get ages in the [30, 59] and [60,74] intervals.

To aggregate names in an array, we use the groupArray aggregate function. It takes one argument. In our case, it's the name column. The groupArrayResample function should use the age column to aggregate names by age. To define the required intervals, we pass 30, 75, 30 as arguments into the groupArrayResample function:

SELECT groupArrayResample(30, 75, 30)(name, age) FROM employee_data

┌─groupArrayResample(30, 75, 30)(name, age)─────┐
│ [['Alice','Mary','Evelyn'],['David','Brian']] │
└───────────────────────────────────────────────┘

See also​