Constant KAFKA_TIMESTAMP_COLUMN_NAME 

pub const KAFKA_TIMESTAMP_COLUMN_NAME: &str = "_rw_kafka_timestamp";

For kafka source, we attach a hidden column KAFKA_TIMESTAMP_COLUMN_NAME to it, so that we can limit the timestamp range when querying it directly with batch query. The column type is crate::types::DataType::Timestamptz. For more details, please refer to this rfc.