I have Kafka engine table, MV and destination MT table.
Kafka table create query:
```
CREATE TABLE abc.Kafka_TestSource (...) 
ENGINE = Kafka 
SETTINGS kafka_broker_list = '...'
, format_avro_schema_registry_url = '...'
, kafka_topic_list = '...'
, kafka_group_name = 'Test_KafkaToCH'
, kafka_format = 'AvroConfluent'
, kafka_num_consumers = 1
, kafka_thread_per_consumer = 0
```

Due to Grafana consumer group stats dashboard it reads the topic with average speed ~11k records per second.
I recreate Kafka table with `kafka_num_consumers = 9` (kafka topic has 9 partitions) and it still reads with the same speed ~11k rps, **system.metrics** also shows that only 1 pool is being used:
```
SELECT
    m.metric,
    m.value
FROM system.metrics AS m
WHERE m.metric LIKE 'Background%'

Query id: 831a9285-e91b-45a4-8a1b-6b536d6860fe

┌─metric──────────────────────────────────┬─value─┐
│ BackgroundMergesAndMutationsPoolTask    │     4 │
│ BackgroundFetchesPoolTask               │     0 │
│ BackgroundCommonPoolTask                │     0 │
│ BackgroundMovePoolTask                  │     0 │
│ BackgroundSchedulePoolTask              │     0 │
│ BackgroundBufferFlushSchedulePoolTask   │     0 │
│ BackgroundDistributedSchedulePoolTask   │     0 │
│ BackgroundMessageBrokerSchedulePoolTask │     1 │
└─────────────────────────────────────────┴───────┘
```

If I set `kafka_thread_per_consumer = 1` then **BackgroundMessageBrokerSchedulePoolTask** in **system.metrics** raises up to 9 and read speed in Grafana raises up to 100 records per second:
![pic](https://github.com/sokolov-alexey/pics/blob/main/ch_kafka.PNG?raw=true)
1. `kafka_num_consumers = 1, kafka_thread_per_consumer = 0`
2. `kafka_num_consumers = 9, kafka_thread_per_consumer = 0`
3. `kafka_num_consumers = 9, kafka_thread_per_consumer = 1`

Why `kafka_num_consumers` doesn't work without `kafka_thread_per_consumer`?

ClickHouse 21.11.4.14.
{
    auto result = Names {
        "_topic",
        "_key",
        "_offset",
        "_partition",
        "_timestamp",
        "_timestamp_ms",
        "_headers.name",
        "_headers.value",
    };
    if (kafka_settings->kafka_handle_error_mode == HandleKafkaErrorMode::STREAM)
    {
        result.push_back({"_raw_message"});
        result.push_back({"_error"});
    }
    return result;
}
}