Clickhouse: Expose Kafka message's key and topic via virtual columns in tables with Kafka engines

[abyss7]

The column for key should be named as _key - if the name is taken by user, then we add the number suffix until we get unused column name: e.g. _key1, _key2, etc.
The column for topic should be named as _topic and uses the same number-suffix logic.

If the Kafka message contains multiple rows, then virtual values are the same for all these rows.

Both virtual columns have type String.

Syntax example:

CREATE TABLE kafka (a UInt8, b String) ENGINE Kafka() SETTINGS …;
SELECT a, b, _topic, _key FROM kafka LIMIT 1;

[filimonov]

Maybe also partition and offset?

[inv2004]

Please do not forget _timestamp

[abyss7]

topic, offset, key columns are available in v19.11+

[abyss7]

Resolved in v19.14+