Kafka Engine, Materialized view and storage

[Loïc Girault]

In Clickhouse's doc for Kafka Engine the following example is given :

CREATE TABLE queue (
  timestamp UInt64,
  level String,
  message String
) ENGINE = Kafka('localhost:9092', 'topic', 'group1', 'JSONEachRow');

CREATE TABLE daily (
  day Date,
  level String,
  total UInt64
) ENGINE = SummingMergeTree(day, (day, level), 8192);

CREATE MATERIALIZED VIEW consumer TO daily
  AS SELECT toDate(toDateTime(timestamp)) AS day, level, count() as total
  FROM queue GROUP BY day, level;

SELECT level, sum(total) FROM daily GROUP BY level;

In addition of the consumer table (`queue`) there is a table and a materialized view. In the  query language section , it is said that "Materialized views store data transformed by the corresponding SELECT query".

So my question is, is the data stored twice ? My intuitition is that obviously not but I'd like to have confirmation.

[Mikhail Filimonov]

That materialized view consumer uses daily table as a storage. That syntax is not in documentation yet.

Please check: https://github.com/yandex/ClickHouse/blob/master/CHANGELOG.md

Added the CREATE MATERIALIZED VIEW x TO y query (specifies an existing table for storing the data of a materialized view).

Proabably it was introduced to make it possible to insert materialized view result in some table with aggregation. Like SummingMergeTree in that example.