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maxMap

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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.

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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.
Query
CREATE TABLE metrics(
    date Date,
    timeslot DateTime,
    status Map(String, UInt64)
) ENGINE = MergeTree
ORDER BY ();

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
Response
   ┌────────────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} │
   └─────────────────────┴──────────────────────────────────────┘

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See also