Add support for ALTER TABLE MODIFY COMMENT

docs/en/sql-reference/statements/alter/comment.md

@@ -57,3 +57,9 @@ Output of a removed comment:
 │         │
 └─────────┘
 ```
+
+**Caveats**
+
+For Replicated tables, the comment can be different on different replicas. Modifying the comment applies to a single replica.
+
+The feature is available since version 23.9. It does not work in previous ClickHouse versions.

src/Interpreters/InterpreterAlterQuery.cpp

@@ -156,10 +156,10 @@ BlockIO InterpreterAlterQuery::executeToTable(const ASTAlterQuery & alter)
     if (typeid_cast<DatabaseReplicated *>(database.get()))
     {
         int command_types_count = !mutation_commands.empty() + !partition_commands.empty() + !alter_commands.empty();
-        bool mixed_settings_amd_metadata_alter = alter_commands.hasSettingsAlterCommand() && !alter_commands.isSettingsAlter();
+        bool mixed_settings_amd_metadata_alter = alter_commands.hasNonReplicatedAlterCommand() && !alter_commands.areNonReplicatedAlterCommands();
         if (1 < command_types_count || mixed_settings_amd_metadata_alter)
             throw Exception(ErrorCodes::NOT_IMPLEMENTED, "For Replicated databases it's not allowed "
-                                                         "to execute ALTERs of different types in single query");
+                                                         "to execute ALTERs of different types (replicated and non replicated) in single query");
     }
 
     if (mutation_commands.hasNonEmptyMutationCommands())

src/Parsers/ParserCreateQuery.cpp

@@ -526,6 +526,7 @@ bool ParserStorage::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
 
         break;
     }
+
     // If any part of storage definition is found create storage node
     if (!storage_like)
         return false;

src/Storages/AlterCommands.cpp

@@ -1350,9 +1350,14 @@ void AlterCommands::validate(const StoragePtr & table, ContextPtr context) const
     validateColumnsDefaultsAndGetSampleBlock(default_expr_list, all_columns.getAll(), context);
 }
 
-bool AlterCommands::hasSettingsAlterCommand() const
+bool AlterCommands::hasNonReplicatedAlterCommand() const
 {
-    return std::any_of(begin(), end(), [](const AlterCommand & c) { return c.isSettingsAlter(); });
+    return std::any_of(begin(), end(), [](const AlterCommand & c) { return c.isSettingsAlter() || c.isCommentAlter(); });
+}
+
+bool AlterCommands::areNonReplicatedAlterCommands() const
+{
+    return std::all_of(begin(), end(), [](const AlterCommand & c) { return c.isSettingsAlter() || c.isCommentAlter(); });
 }
 
 bool AlterCommands::isSettingsAlter() const

src/Storages/AlterCommands.h

@@ -196,8 +196,11 @@ class AlterCommands : public std::vector<AlterCommand>
     /// Commands have to be prepared before apply.
     void apply(StorageInMemoryMetadata & metadata, ContextPtr context) const;
 
-    /// At least one command modify settings.
-    bool hasSettingsAlterCommand() const;
+    /// At least one command modify settings or comments.
+    bool hasNonReplicatedAlterCommand() const;
+
+    /// All commands modify settings or comments.
+    bool areNonReplicatedAlterCommands() const;
 
     /// All commands modify settings only.
     bool isSettingsAlter() const;

src/Storages/MergeTree/registerStorageMergeTree.cpp

@@ -80,6 +80,7 @@ ORDER BY expr
 [SAMPLE BY expr]
 [TTL expr [DELETE|TO DISK 'xxx'|TO VOLUME 'xxx'], ...]
 [SETTINGS name=value, ...]
+[COMMENT 'comment']
 
 See details in documentation: https://clickhouse.com/docs/en/engines/table-engines/mergetree-family/mergetree/. Other engines of the family support different syntax, see details in the corresponding documentation topics.
 

src/Storages/StorageMergeTree.cpp

@@ -332,6 +332,11 @@ void StorageMergeTree::alter(
         changeSettings(new_metadata.settings_changes, table_lock_holder);
         DatabaseCatalog::instance().getDatabase(table_id.database_name)->alterTable(local_context, table_id, new_metadata);
     }
+    else if (commands.isCommentAlter())
+    {
+        setInMemoryMetadata(new_metadata);
+        DatabaseCatalog::instance().getDatabase(table_id.database_name)->alterTable(local_context, table_id, new_metadata);
+    }
     else
     {
         if (!maybe_mutation_commands.empty() && maybe_mutation_commands.containBarrierCommand())

src/Storages/StorageReplicatedMergeTree.cpp

@@ -5694,6 +5694,17 @@ void StorageReplicatedMergeTree::alter(
         return;
     }
 
+    if (commands.isCommentAlter())
+    {
+        StorageInMemoryMetadata future_metadata = getInMemoryMetadata();
+        commands.apply(future_metadata, query_context);
+
+        setInMemoryMetadata(future_metadata);
+
+        DatabaseCatalog::instance().getDatabase(table_id.database_name)->alterTable(query_context, table_id, future_metadata);
+        return;
+    }
+
     auto ast_to_str = [](ASTPtr query) -> String
     {
         if (!query)
@@ -5763,12 +5774,27 @@ void StorageReplicatedMergeTree::alter(
         String new_columns_str = future_metadata.columns.toString();
         ops.emplace_back(zkutil::makeSetRequest(fs::path(zookeeper_path) / "columns", new_columns_str, -1));
 
-        if (ast_to_str(current_metadata->settings_changes) != ast_to_str(future_metadata.settings_changes))
+        bool settings_are_changed = (ast_to_str(current_metadata->settings_changes) != ast_to_str(future_metadata.settings_changes));
+        bool comment_is_changed = (current_metadata->comment != future_metadata.comment);
+
+        if (settings_are_changed || comment_is_changed)
         {
-            /// Just change settings
             StorageInMemoryMetadata metadata_copy = *current_metadata;
-            metadata_copy.settings_changes = future_metadata.settings_changes;
-            changeSettings(metadata_copy.settings_changes, table_lock_holder);
+
+            if (settings_are_changed)
+            {
+                /// Just change settings
+                metadata_copy.settings_changes = future_metadata.settings_changes;
+                changeSettings(metadata_copy.settings_changes, table_lock_holder);
+            }
+
+            /// The comment is not replicated as of today, but we can implement it later.
+            if (comment_is_changed)
+            {
+                metadata_copy.setComment(future_metadata.comment);
+                setInMemoryMetadata(metadata_copy);
+            }
+
             DatabaseCatalog::instance().getDatabase(table_id.database_name)->alterTable(query_context, table_id, metadata_copy);
         }
 

tests/queries/0_stateless/00725_comment_columns_long.sql

@@ -1,4 +1,5 @@
--- Tags: long
+-- Tags: long, no-replicated-database
+-- Tag no-replicated-database: Unsupported type of ALTER query
 
 DROP TABLE IF EXISTS check_query_comment_column;
 

tests/queries/0_stateless/02020_alter_table_modify_comment.reference

@@ -124,21 +124,21 @@ CREATE TABLE default.comment_test_table\n(\n    `k` UInt64,\n    `s` String\n)\n
 comment=	Test table with comment
 
 change a comment
-CREATE TABLE default.comment_test_table\n(\n    `k` UInt64,\n    `s` String\n)\nENGINE = ReplicatedMergeTree(\'/clickhouse/02020_alter_table_modify_comment_default\', \'1\')\nORDER BY k\nSETTINGS index_granularity = 8192\nCOMMENT \'Test table with comment\'
-comment=	Test table with comment
+CREATE TABLE default.comment_test_table\n(\n    `k` UInt64,\n    `s` String\n)\nENGINE = ReplicatedMergeTree(\'/clickhouse/02020_alter_table_modify_comment_default\', \'1\')\nORDER BY k\nSETTINGS index_granularity = 8192\nCOMMENT \'new comment on a table\'
+comment=	new comment on a table
 
 remove a comment
-CREATE TABLE default.comment_test_table\n(\n    `k` UInt64,\n    `s` String\n)\nENGINE = ReplicatedMergeTree(\'/clickhouse/02020_alter_table_modify_comment_default\', \'1\')\nORDER BY k\nSETTINGS index_granularity = 8192\nCOMMENT \'Test table with comment\'
-comment=	Test table with comment
+CREATE TABLE default.comment_test_table\n(\n    `k` UInt64,\n    `s` String\n)\nENGINE = ReplicatedMergeTree(\'/clickhouse/02020_alter_table_modify_comment_default\', \'1\')\nORDER BY k\nSETTINGS index_granularity = 8192
+comment=	
 
 add a comment back
-CREATE TABLE default.comment_test_table\n(\n    `k` UInt64,\n    `s` String\n)\nENGINE = ReplicatedMergeTree(\'/clickhouse/02020_alter_table_modify_comment_default\', \'1\')\nORDER BY k\nSETTINGS index_granularity = 8192\nCOMMENT \'Test table with comment\'
-comment=	Test table with comment
+CREATE TABLE default.comment_test_table\n(\n    `k` UInt64,\n    `s` String\n)\nENGINE = ReplicatedMergeTree(\'/clickhouse/02020_alter_table_modify_comment_default\', \'1\')\nORDER BY k\nSETTINGS index_granularity = 8192\nCOMMENT \'another comment on a table\'
+comment=	another comment on a table
 
 detach table
-CREATE TABLE default.comment_test_table\n(\n    `k` UInt64,\n    `s` String\n)\nENGINE = ReplicatedMergeTree(\'/clickhouse/02020_alter_table_modify_comment_default\', \'1\')\nORDER BY k\nSETTINGS index_granularity = 8192\nCOMMENT \'Test table with comment\'
+CREATE TABLE default.comment_test_table\n(\n    `k` UInt64,\n    `s` String\n)\nENGINE = ReplicatedMergeTree(\'/clickhouse/02020_alter_table_modify_comment_default\', \'1\')\nORDER BY k\nSETTINGS index_granularity = 8192\nCOMMENT \'another comment on a table\'
 
 re-attach table
-CREATE TABLE default.comment_test_table\n(\n    `k` UInt64,\n    `s` String\n)\nENGINE = ReplicatedMergeTree(\'/clickhouse/02020_alter_table_modify_comment_default\', \'1\')\nORDER BY k\nSETTINGS index_granularity = 8192\nCOMMENT \'Test table with comment\'
-comment=	Test table with comment
+CREATE TABLE default.comment_test_table\n(\n    `k` UInt64,\n    `s` String\n)\nENGINE = ReplicatedMergeTree(\'/clickhouse/02020_alter_table_modify_comment_default\', \'1\')\nORDER BY k\nSETTINGS index_granularity = 8192\nCOMMENT \'another comment on a table\'
+comment=	another comment on a table
 

tests/queries/0_stateless/02792_alter_table_modify_comment.reference

@@ -0,0 +1,20 @@
+Hello
+World
+Hello
+World
+Hello
+World
+Hello
+World
+Hello
+World
+
+World
+
+World
+
+World
+
+World
+
+World

tests/queries/0_stateless/02792_alter_table_modify_comment.sql

@@ -0,0 +1,72 @@
+-- Tags: no-replicated-database
+-- Tag no-replicated-database: Unsupported type of ALTER query
+
+DROP TABLE IF EXISTS t;
+
+# Memory, MergeTree, and ReplicatedMergeTree
+
+CREATE TABLE t (x UInt8) ENGINE = Memory COMMENT 'Hello';
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+ALTER TABLE t MODIFY COMMENT 'World';
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+DROP TABLE t;
+
+CREATE TABLE t (x UInt8) ENGINE = MergeTree ORDER BY () COMMENT 'Hello';
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+ALTER TABLE t MODIFY COMMENT 'World';
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+DROP TABLE t;
+
+# The case when there are many operations in one ALTER
+
+CREATE TABLE t (x UInt8) ENGINE = MergeTree ORDER BY () COMMENT 'Hello';
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+ALTER TABLE t MODIFY COMMENT 'World', MODIFY COLUMN x UInt16;
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+DROP TABLE t;
+
+# Note that the table comment is not replicated. We can implement it later.
+
+CREATE TABLE t (x UInt8) ENGINE = ReplicatedMergeTree('/clickhouse/tables/{database}/test_comment_table1/t', '1') ORDER BY () COMMENT 'Hello';
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+ALTER TABLE t MODIFY COMMENT 'World';
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+DROP TABLE t SYNC;
+
+CREATE TABLE t (x UInt8) ENGINE = ReplicatedMergeTree('/clickhouse/tables/{database}/test_comment_table2/t', '1') ORDER BY () COMMENT 'Hello';
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+ALTER TABLE t MODIFY COMMENT 'World', MODIFY COLUMN x UInt16;
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+DROP TABLE t SYNC;
+
+# The cases when there is no comment on creation
+
+CREATE TABLE t (x UInt8) ENGINE = Memory;
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+ALTER TABLE t MODIFY COMMENT 'World';
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+DROP TABLE t;
+
+CREATE TABLE t (x UInt8) ENGINE = MergeTree ORDER BY ();
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+ALTER TABLE t MODIFY COMMENT 'World';
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+DROP TABLE t;
+
+CREATE TABLE t (x UInt8) ENGINE = MergeTree ORDER BY ();
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+ALTER TABLE t MODIFY COMMENT 'World', MODIFY COLUMN x UInt16;
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+DROP TABLE t;
+
+CREATE TABLE t (x UInt8) ENGINE = ReplicatedMergeTree('/clickhouse/tables/{database}/test_comment_table3/t', '1') ORDER BY ();
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+ALTER TABLE t MODIFY COMMENT 'World';
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+DROP TABLE t SYNC;
+
+CREATE TABLE t (x UInt8) ENGINE = ReplicatedMergeTree('/clickhouse/tables/{database}/test_comment_table4/t', '1') ORDER BY ();
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+ALTER TABLE t MODIFY COMMENT 'World', MODIFY COLUMN x UInt16;
+SELECT comment FROM system.tables WHERE database = currentDatabase() AND table = 't';
+DROP TABLE t SYNC;