KafkaEngine: Allow usage of Alias column type

docs/en/engines/table-engines/integrations/kafka.md

@@ -19,8 +19,8 @@ Kafka lets you:
 ``` sql
 CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
 (
-    name1 [type1],
-    name2 [type2],
+    name1 [type1] [ALIAS expr1],
+    name2 [type2] [ALIAS expr2],
     ...
 ) ENGINE = Kafka()
 SETTINGS

src/Storages/ColumnsDescription.cpp

@@ -383,15 +383,6 @@ NamesAndTypesList ColumnsDescription::getEphemeral() const
     return ret;
 }
 
-NamesAndTypesList ColumnsDescription::getWithDefaultExpression() const
-{
-    NamesAndTypesList ret;
-    for (const auto & col : columns)
-        if (col.default_desc.expression)
-            ret.emplace_back(col.name, col.type);
-    return ret;
-}
-
 NamesAndTypesList ColumnsDescription::getAll() const
 {
     NamesAndTypesList ret;

src/Storages/ColumnsDescription.h

@@ -132,7 +132,6 @@ class ColumnsDescription : public IHints<1, ColumnsDescription>
     NamesAndTypesList getInsertable() const; /// ordinary + ephemeral
     NamesAndTypesList getAliases() const;
     NamesAndTypesList getEphemeral() const;
-    NamesAndTypesList getWithDefaultExpression() const; // columns with default expression, for example set by `CREATE TABLE` statement
     NamesAndTypesList getAllPhysical() const; /// ordinary + materialized.
     NamesAndTypesList getAll() const; /// ordinary + materialized + aliases + ephemeral
     /// Returns .size0/.null/...

src/Storages/Kafka/StorageKafka.cpp

@@ -41,6 +41,7 @@
 #include <Common/setThreadName.h>
 #include <Formats/FormatFactory.h>
 
+#include "Storages/ColumnDefault.h"
 #include "config_version.h"
 
 #include <Common/CurrentMetrics.h>
@@ -966,9 +967,18 @@ void registerStorageKafka(StorageFactory & factory)
         {
             throw Exception(ErrorCodes::BAD_ARGUMENTS, "kafka_poll_max_batch_size can not be lower than 1");
         }
-        if (args.columns.getOrdinary() != args.columns.getAll() || !args.columns.getWithDefaultExpression().empty())
+        NamesAndTypesList supported_columns;
+        for (const auto & column : args.columns)
         {
-            throw Exception(ErrorCodes::BAD_ARGUMENTS, "KafkaEngine doesn't support DEFAULT/MATERIALIZED/EPHEMERAL/ALIAS expressions for columns. "
+            if (column.default_desc.kind == ColumnDefaultKind::Alias)
+                supported_columns.emplace_back(column.name, column.type);
+            if (column.default_desc.kind == ColumnDefaultKind::Default && !column.default_desc.expression)
+                supported_columns.emplace_back(column.name, column.type);
+        }
+        // Kafka engine allows only ordinary columns without default expression or alias columns.
+        if (args.columns.getAll() != supported_columns)
+        {
+            throw Exception(ErrorCodes::BAD_ARGUMENTS, "KafkaEngine doesn't support DEFAULT/MATERIALIZED/EPHEMERAL expressions for columns. "
                                                        "See https://clickhouse.com/docs/en/engines/table-engines/integrations/kafka/#configuration");
         }
 

tests/integration/test_storage_kafka/test.py

@@ -285,11 +285,11 @@ def avro_confluent_message(schema_registry_client, value):
 # Tests
 
 
-def test_kafka_prohibited_column_types(kafka_cluster):
+def test_kafka_column_types(kafka_cluster):
     def assert_returned_exception(e):
         assert e.value.returncode == 36
         assert (
-            "KafkaEngine doesn't support DEFAULT/MATERIALIZED/EPHEMERAL/ALIAS expressions for columns."
+            "KafkaEngine doesn't support DEFAULT/MATERIALIZED/EPHEMERAL expressions for columns."
             in str(e.value)
         )
 
@@ -314,17 +314,39 @@ def assert_returned_exception(e):
     assert_returned_exception(exception)
 
     # check ALIAS
-    with pytest.raises(QueryRuntimeException) as exception:
-        instance.query(
-            """
+    instance.query(
+        """
                 CREATE TABLE test.kafka (a Int, b String Alias toString(a))
                 ENGINE = Kafka('{kafka_broker}:19092', '{kafka_topic_new}', '{kafka_group_name_new}', '{kafka_format_json_each_row}', '\\n')
+                SETTINGS kafka_commit_on_select = 1;
                 """
-        )
-    assert_returned_exception(exception)
+    )
+    messages = []
+    for i in range(5):
+        messages.append(json.dumps({"a": i}))
+    kafka_produce(kafka_cluster, "new", messages)
+    result = ""
+    expected = TSV(
+        """
+0\t0
+1\t1
+2\t2
+3\t3
+4\t4
+                              """
+    )
+    retries = 50
+    while retries > 0:
+        result += instance.query("SELECT a, b FROM test.kafka", ignore_error=True)
+        if TSV(result) == expected:
+            break
+        retries -= 1
+
+    assert TSV(result) == expected
+
+    instance.query("DROP TABLE test.kafka SYNC")
 
     # check MATERIALIZED
-    # check ALIAS
     with pytest.raises(QueryRuntimeException) as exception:
         instance.query(
             """