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maxMap

Description​

The Map combinator can be applied to the max function to calculate the maximum value in a Map according to each key, using the maxMap aggregate combinator function.

Example usage​

In this example, we'll create a table that stores status codes and their counts for different timeslots, where each row contains a Map of status codes to their corresponding counts. We'll use maxMap to find the maximum count for each status code within each timeslot.

CREATE TABLE metrics(
    date Date,
    timeslot DateTime,
    status Map(String, UInt64)
) ENGINE = Log;

INSERT INTO metrics VALUES
    ('2000-01-01', '2000-01-01 00:00:00', (['a', 'b', 'c'], [15, 25, 35])),
    ('2000-01-01', '2000-01-01 00:00:00', (['c', 'd', 'e'], [45, 55, 65])),
    ('2000-01-01', '2000-01-01 00:01:00', (['d', 'e', 'f'], [75, 85, 95])),
    ('2000-01-01', '2000-01-01 00:01:00', (['f', 'g', 'g'], [105, 115, 125]));

SELECT
    timeslot,
    maxMap(status),
FROM metrics
GROUP BY timeslot;

The maxMap function will find the maximum count for each status code within each timeslot. For example:

  • In timeslot '2000-01-01 00:00:00':
    • Status 'a': 15
    • Status 'b': 25
    • Status 'c': max(35, 45) = 45
    • Status 'd': 55
    • Status 'e': 65
  • In timeslot '2000-01-01 00:01:00':
    • Status 'd': 75
    • Status 'e': 85
    • Status 'f': max(95, 105) = 105
    • Status 'g': max(115, 125) = 125
   ┌────────────timeslot─┬─maxMap(status)───────────────────────┐
1. │ 2000-01-01 00:01:00 │ {'d':75,'e':85,'f':105,'g':125}      │
2. │ 2000-01-01 00:00:00 │ {'a':15,'b':25,'c':45,'d':55,'e':65} │
   └─────────────────────┴──────────────────────────────────────┘

See also​