kafka_dead_letter_queue: Nullable, forgotten doc added

docs/en/operations/system-tables/dead_letter_queue.md

@@ -0,0 +1,87 @@
+---
+slug: /en/operations/system-tables/dead_letter_queue
+---
+# dead_letter_queue
+
+Contains information about messages received via a stream engine and parsed with an errors. Currently implemented for Kafka.
+
+Logging is controlled by `dead_letter_queue` of `kafka_handle_error_mode` setting.
+
+The flushing period of data is set in `flush_interval_milliseconds` parameter of the [dead_letter_queue](../../operations/server-configuration-parameters/settings.md#server_configuration_parameters-dead_letter_queue) server settings section. To force flushing, use the [SYSTEM FLUSH LOGS](../../sql-reference/statements/system.md#query_language-system-flush_logs) query.
+
+ClickHouse does not delete data from the table automatically. See [Introduction](../../operations/system-tables/index.md#system-tables-introduction) for more details.
+
+Columns:
+
+- `stream_type` ([Enum8](../../sql-reference/data-types/enum.md)) - Stream type. Possible values: 'Kafka'.
+- `event_date` ([Date](../../sql-reference/data-types/date.md)) - Message consuming date.
+- `event_time` ([DateTime](../../sql-reference/data-types/datetime.md)) - Message consuming date and time.
+- `event_time_microseconds`([DateTime64](../../sql-reference/data-types/datetime64.md)) - Message consuming time with microseconds precision.
+- `database_name` ([LowCardinality(String)](../../sql-reference/data-types/string.md)) - ClickHouse database Kafka table belongs to.
+- `table_name` ([LowCardinality(String)](../../sql-reference/data-types/string.md)) - ClickHouse table name.
+- `topic_name` ([Nullable(String)](../../sql-reference/data-types/nullable.md)) - Topic name.
+- `partition` ([Nullable(UInt64)](../../sql-reference/data-types/nullable.md)) - Partition.
+- `offset` ([Nullable(UInt64)](../../sql-reference/data-types/nullable.md)) - Offset.
+- `raw_message` ([String](../../sql-reference/data-types/string.md)) - Message body.
+- `error` ([String](../../sql-reference/data-types/string.md)) - Error text.
+
+**Example**
+
+Query:
+
+``` sql
+SELECT * FROM system.asynchronous_insert_log LIMIT 1 \G;
+```
+
+Result:
+
+``` text
+Row 1:
+──────
+stream_type:             Kafka
+event_date:              2024-08-26
+event_time:              2024-08-26 07:49:20
+event_time_microseconds: 2024-08-26 07:49:20.268091
+database_name:           default
+table_name:              kafka
+topic_name:              CapnProto_dead_letter_queue_err
+partition:               0
+offset:                  0
+raw_message:             qwertyuiop
+error:                   Message has too many segments. Most likely, data was corrupted: (at row 1)
+
+
+Row 2:
+──────
+stream_type:             Kafka
+event_date:              2024-08-26
+event_time:              2024-08-26 07:49:20
+event_time_microseconds: 2024-08-26 07:49:20.268361
+database_name:           default
+table_name:              kafka
+topic_name:              CapnProto_dead_letter_queue_err
+partition:               0
+offset:                  0
+raw_message:             asdfghjkl
+error:                   Message has too many segments. Most likely, data was corrupted: (at row 1)
+
+
+Row 3:
+──────
+stream_type:             Kafka
+event_date:              2024-08-26
+event_time:              2024-08-26 07:49:20
+event_time_microseconds: 2024-08-26 07:49:20.268604
+database_name:           default
+table_name:              kafka
+topic_name:              CapnProto_dead_letter_queue_err
+partition:               0
+offset:                  0
+raw_message:             zxcvbnm
+error:                   Message has too many segments. Most likely, data was corrupted: (at row 1)
+```
+
+**See Also**
+
+- [Kafka](../../engines/table-engines/integrations/kafka) - Kafka Engine
+- [system.kafka_consumers](../../operations/system-tables/kafka_consumers.md#system_tables-kafka_consumers) — Description of the `kafka_consumers` system table which contains information like statistics and errors about Kafka consumers.

src/Interpreters/DeadLetterQueue.cpp

@@ -7,7 +7,7 @@
 #include <DataTypes/DataTypeLowCardinality.h>
 #include <DataTypes/DataTypeString.h>
 #include <DataTypes/DataTypeEnum.h>
-
+#include <DataTypes/DataTypeNullable.h>
 
 namespace DB
 {
@@ -29,9 +29,9 @@ ColumnsDescription DeadLetterQueueElement::getColumnsDescription()
         {"event_time_microseconds", std::make_shared<DataTypeDateTime64>(6), "Query starting time with microseconds precision."},
         {"database_name", low_cardinality_string, "ClickHouse database Kafka table belongs to."},
         {"table_name", low_cardinality_string, "ClickHouse table name."},
-        {"topic_name", low_cardinality_string, "Topic name."},
+        {"topic_name", std::make_shared<DataTypeNullable>(std::make_shared<DataTypeString>()), "Topic name."},
-        {"partition", std::make_shared<DataTypeUInt64>(), "Partition."},
+        {"partition", std::make_shared<DataTypeNullable>(std::make_shared<DataTypeUInt64>()), "Partition."},
-        {"offset", std::make_shared<DataTypeUInt64>(), "Offset."},
+        {"offset", std::make_shared<DataTypeNullable>(std::make_shared<DataTypeUInt64>()), "Offset."},
         {"raw_message", std::make_shared<DataTypeString>(), "Message body."},
         {"error", std::make_shared<DataTypeString>(), "Error text."}
     };