ClickHouse/docs/en/operations/table_engines/aggregatingmergetree.md

AggregatingMergeTree

This engine differs from MergeTree in that the merge combines the states of aggregate functions stored in the table for rows with the same primary key value.

For this to work, it uses the AggregateFunction data type, as well as -State and -Merge modifiers for aggregate functions. Let's examine it more closely.

There is an AggregateFunction data type. It is a parametric data type. As parameters, the name of the aggregate function is passed, then the types of its arguments.

Examples:

CREATE TABLE t
(
    column1 AggregateFunction(uniq, UInt64),
    column2 AggregateFunction(anyIf, String, UInt8),
    column3 AggregateFunction(quantiles(0.5, 0.9), UInt64)
) ENGINE = ...

This type of column stores the state of an aggregate function.

To get this type of value, use aggregate functions with the State suffix.

Example:uniqState(UserID), quantilesState(0.5, 0.9)(SendTiming)

In contrast to the corresponding uniq and quantiles functions, these functions return the state, rather than the prepared value. In other words, they return an AggregateFunction type value.

An AggregateFunction type value can't be output in Pretty formats. In other formats, these types of values are output as implementation-specific binary data. The AggregateFunction type values are not intended for output or saving in a dump.

The only useful thing you can do with AggregateFunction type values is to combine the states and get a result, which essentially means to finish aggregation. Aggregate functions with the 'Merge' suffix are used for this purpose. Example: uniqMerge(UserIDState), where UserIDState has the AggregateFunction type.

In other words, an aggregate function with the 'Merge' suffix takes a set of states, combines them, and returns the result. As an example, these two queries return the same result:

SELECT uniq(UserID) FROM table

SELECT uniqMerge(state) FROM (SELECT uniqState(UserID) AS state FROM table GROUP BY RegionID)

There is an AggregatingMergeTree engine. Its job during a merge is to combine the states of aggregate functions from different table rows with the same primary key value.

You can't use a normal INSERT to insert a row in a table containing AggregateFunction columns, because you can't explicitly define the AggregateFunction value. Instead, use INSERT SELECT with -State aggregate functions for inserting data.

With SELECT from an AggregatingMergeTree table, use GROUP BY and aggregate functions with the '-Merge' modifier in order to complete data aggregation.

You can use AggregatingMergeTree tables for incremental data aggregation, including for aggregated materialized views.

Example:

Create an AggregatingMergeTree materialized view that watches the test.visits table:

CREATE MATERIALIZED VIEW test.basic
ENGINE = AggregatingMergeTree(StartDate, (CounterID, StartDate), 8192)
AS SELECT
    CounterID,
    StartDate,
    sumState(Sign)    AS Visits,
    uniqState(UserID) AS Users
FROM test.visits
GROUP BY CounterID, StartDate;

Insert data in the test.visits table. Data will also be inserted in the view, where it will be aggregated:

INSERT INTO test.visits ...

Perform SELECT from the view using GROUP BY in order to complete data aggregation:

SELECT
    StartDate,
    sumMerge(Visits) AS Visits,
    uniqMerge(Users) AS Users
FROM test.basic
GROUP BY StartDate
ORDER BY StartDate;

You can create a materialized view like this and assign a normal view to it that finishes data aggregation.

Note that in most cases, using AggregatingMergeTree is not justified, since queries can be run efficiently enough on non-aggregated data.

Original article