Hi!

Is this the same question: https://stackoverflow.com/questions/78468438/producing-messages-from-clickhouse-materialized-view-to-kafka-topic-by-kafka-eng ?

Based on the code implementation, _key column needs to be a String type. You may try to convert to string using toString() or using some other String type column.

From current source code _key will work on producer side too:

KafkaProducer.cpp:27-34 — on construction, scans the block header: if there's a column named _key and it's String, it records that column's index.

KafkaProducer.cpp:52-57 — on produce(), if that index is set, extracts that column's value at the row position and passes it to builder.key(...) → that's the Kafka message key.

StorageKafka.cpp:267 — the header is getSampleBlockNonMaterialized(), i.e., regular columns including _key. So _key participates in BOTH the producer's key handling AND the format serialization of the value.