avg_merge_state ​

avg_merge_state merges average states and returns another state. Use it when a calculation has several stages, such as combining per-batch states before combining them across a larger period.

Create batch states ​

CREATE TABLE readings(batch UInt8, value Int64);
INSERT INTO readings VALUES (1, 10), (1, 20), (2, 90);
CREATE TABLE partials(batch UInt8, state AggregateFunction(avg, Int64));
INSERT INTO partials
SELECT batch, avg_state(value)
FROM readings
GROUP BY batch;

Retain the combined state ​

CREATE TABLE combined(state AggregateFunction(avg, Int64));
INSERT INTO combined SELECT avg_merge_state(state) FROM partials;

Finalize at the boundary ​

SELECT finalize_aggregation(state)
FROM combined
FORMAT csv;

40.0

The merged state still retains the contribution count needed by another avg_merge or avg_merge_state. Finalizing a state reads it without combining other rows.

See avg_state, avg_merge and aggregate combinators.