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minSimpleState

Description​

The SimpleState combinator can be applied to the min function to return the minimum value across all input values. It returns the result with type SimpleAggregateFunction.

Example usage​

Let's look at a practical example using a table that tracks daily temperature readings. For each location, we want to maintain the lowest temperature recorded. Using the SimpleAggregateFunction type with min automatically updates the stored value when a lower temperature is encountered.

Create the source table for raw temperature readings:

CREATE TABLE raw_temperature_readings
(
    location_id UInt32,
    location_name String,
    temperature Int32,
    recorded_at DateTime DEFAULT now()
)
    ENGINE = MergeTree()
ORDER BY (location_id, recorded_at);

Create the aggregate table that will store the min temperatures:

CREATE TABLE temperature_extremes
(
    location_id UInt32,
    location_name String,
    min_temp SimpleAggregateFunction(min, Int32),  -- Stores minimum temperature
    max_temp SimpleAggregateFunction(max, Int32)   -- Stores maximum temperature
)
ENGINE = AggregatingMergeTree()
ORDER BY location_id;

Create an Incremental materialized view that will act as an insert trigger for inserted data and maintains the minimum, maximum temperatures per location.

CREATE MATERIALIZED VIEW temperature_extremes_mv
TO temperature_extremes
AS SELECT
    location_id,
    location_name,
    minSimpleState(temperature) AS min_temp,     -- Using SimpleState combinator
    maxSimpleState(temperature) AS max_temp      -- Using SimpleState combinator
FROM raw_temperature_readings
GROUP BY location_id, location_name;

Insert some initial temperature readings:

INSERT INTO raw_temperature_readings (location_id, location_name, temperature) VALUES
(1, 'North', 5),
(2, 'South', 15),
(3, 'West', 10),
(4, 'East', 8);

These readings are automatically processed by the materialized view. Let's check the current state:

SELECT
    location_id,
    location_name,
    min_temp,     -- Directly accessing the SimpleAggregateFunction values
    max_temp      -- No need for finalization function with SimpleAggregateFunction
FROM temperature_extremes
ORDER BY location_id;

┌─location_id─┬─location_name─┬─min_temp─┬─max_temp─┐
│           1 │ North         │        5 │        5 │
│           2 │ South         │       15 │       15 │
│           3 │ West          │       10 │       10 │
│           4 │ East          │        8 │        8 │
└─────────────┴───────────────┴──────────┴──────────┘

Insert some more data:

INSERT INTO raw_temperature_readings (location_id, location_name, temperature) VALUES
    (1, 'North', 3),
    (2, 'South', 18),
    (3, 'West', 10),
    (1, 'North', 8),
    (4, 'East', 2);

View the updated extremes after new data:

SELECT
    location_id,
    location_name,
    min_temp,  
    max_temp
FROM temperature_extremes
ORDER BY location_id;

┌─location_id─┬─location_name─┬─min_temp─┬─max_temp─┐
│           1 │ North         │        3 │        8 │
│           1 │ North         │        5 │        5 │
│           2 │ South         │       18 │       18 │
│           2 │ South         │       15 │       15 │
│           3 │ West          │       10 │       10 │
│           3 │ West          │       10 │       10 │
│           4 │ East          │        2 │        2 │
│           4 │ East          │        8 │        8 │
└─────────────┴───────────────┴──────────┴──────────┘

Notice above that we have two inserted values for each location. This is because parts have not yet been merged (and aggregated by AggregatingMergeTree). To get the final result from the partial states we need to add a GROUP BY:

SELECT
    location_id,
    location_name,
    min(min_temp) AS min_temp,  -- Aggregate across all parts 
    max(max_temp) AS max_temp   -- Aggregate across all parts
FROM temperature_extremes
GROUP BY location_id, location_name
ORDER BY location_id;

We now get the expected result:

┌─location_id─┬─location_name─┬─min_temp─┬─max_temp─┐
│           1 │ North         │        3 │        8 │
│           2 │ South         │       15 │       18 │
│           3 │ West          │       10 │       10 │
│           4 │ East          │        2 │        8 │
└─────────────┴───────────────┴──────────┴──────────┘
Note

With SimpleState, you do not need to use the Merge combinator to combine partial aggregation states.

See also​