Merge pull request #1118 from Altinity/backport/25.3.8/75622

25.3.8 Backport of ClickHouse#75622: Allow altering database comment

Author: zvonand

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

@@ -0,0 +1,55 @@
+---
+description: 'Documentation for ALTER DATABASE ... MODIFY COMMENT Statements'
+slug: /sql-reference/statements/alter/database-comment
+sidebar_position: 51
+sidebar_label: 'UPDATE'
+title: 'ALTER DATABASE ... MODIFY COMMENT Statements'
+---
+
+# ALTER DATABASE ... MODIFY COMMENT
+
+Adds, modifies, or removes comment to the table, regardless if it was set before or not. Comment change is reflected in both [system.databases](/operations/system-tables/databases.md) and `SHOW CREATE DATABASE` query.
+
+**Syntax**
+
+``` sql
+ALTER DATABASE [db].name [ON CLUSTER cluster] MODIFY COMMENT 'Comment'
+```
+
+**Examples**
+
+Creating a DATABASE with comment (for more information, see the [COMMENT](/sql-reference/statements/create/table#comment-clause) clause):
+
+``` sql
+CREATE DATABASE database_with_comment ENGINE = Memory COMMENT 'The temporary database';
+```
+
+Modifying the table comment:
+
+``` sql
+ALTER DATABASE database_with_comment MODIFY COMMENT 'new comment on a database';
+SELECT comment FROM system.databases WHERE name = 'database_with_comment';
+```
+
+Output of a new comment:
+
+```text
+┌─comment─────────────────┐
+│ new comment on database │
+└─────────────────────────┘
+```
+
+Removing the database comment:
+
+``` sql
+ALTER DATABASE database_with_comment MODIFY COMMENT '';
+SELECT comment FROM system.databases WHERE  name = 'database_with_comment';
+```
+
+Output of a removed comment:
+
+```text
+┌─comment─┐
+│         │
+└─────────┘
+```

src/Access/Common/AccessType.h

@@ -43,6 +43,7 @@ enum class AccessType : uint8_t
     M(ALTER_MATERIALIZE_COLUMN, "MATERIALIZE COLUMN", COLUMN, ALTER_COLUMN) \
     M(ALTER_COLUMN, "", GROUP, ALTER_TABLE) /* allow to execute ALTER {ADD|DROP|MODIFY...} COLUMN */\
     M(ALTER_MODIFY_COMMENT, "MODIFY COMMENT", TABLE, ALTER_TABLE) /* modify table comment */\
+    M(ALTER_MODIFY_DATABASE_COMMENT, "MODIFY DATABASE COMMENT", DATABASE, ALTER_DATABASE) /* modify database comment*/\
     \
     M(ALTER_ORDER_BY, "ALTER MODIFY ORDER BY, MODIFY ORDER BY", TABLE, ALTER_INDEX) \
     M(ALTER_SAMPLE_BY, "ALTER MODIFY SAMPLE BY, MODIFY SAMPLE BY", TABLE, ALTER_INDEX) \

src/Databases/DatabaseMemory.cpp

@@ -213,6 +213,11 @@ std::vector<std::pair<ASTPtr, StoragePtr>> DatabaseMemory::getTablesForBackup(co
     return res;
 }
 
+void DatabaseMemory::alterDatabaseComment(const AlterCommand & command)
+{
+    DB::updateDatabaseCommentWithMetadataFile(shared_from_this(), command);
+}
+
 void registerDatabaseMemory(DatabaseFactory & factory)
 {
     auto create_fn = [](const DatabaseFactory::Arguments & args)

src/Databases/DatabaseMemory.h

@@ -52,6 +52,8 @@ class DatabaseMemory final : public DatabaseWithOwnTablesBase
 
     std::vector<std::pair<ASTPtr, StoragePtr>> getTablesForBackup(const FilterByNameFunction & filter, const ContextPtr & local_context) const override;
 
+    void alterDatabaseComment(const AlterCommand & alter_command) override;
+
 private:
     void removeDataPath(ContextPtr local_context);
 

src/Databases/DatabaseOnDisk.cpp

@@ -23,6 +23,7 @@
 #include <Parsers/ASTFunction.h>
 #include <Parsers/ParserCreateQuery.h>
 #include <Parsers/parseQuery.h>
+#include <Storages/AlterCommands.h>
 #include <Storages/IStorage.h>
 #include <Storages/StorageFactory.h>
 #include <TableFunctions/TableFunctionFactory.h>
@@ -916,4 +917,9 @@ void DatabaseOnDisk::checkTableNameLengthUnlocked(const String & database_name_,
     }
 }
 
+void DatabaseOnDisk::alterDatabaseComment(const AlterCommand & command)
+{
+    DB::updateDatabaseCommentWithMetadataFile(shared_from_this(), command);
+}
+
 }

src/Databases/DatabaseOnDisk.h

@@ -12,6 +12,8 @@ namespace DB
 {
 
 class Context;
+struct AlterCommand;
+
 std::pair<String, StoragePtr> createTableFromAST(
     ASTCreateQuery ast_create_query,
     const String & database_name,
@@ -82,6 +84,8 @@ class DatabaseOnDisk : public DatabaseWithOwnTablesBase
 
     void modifySettingsMetadata(const SettingsChanges & settings_changes, ContextPtr query_context);
 
+    void alterDatabaseComment(const AlterCommand & alter_command) override;
+
 protected:
     static constexpr const char * create_suffix = ".tmp";
     static constexpr const char * drop_suffix = ".tmp_drop";

src/Databases/DatabasesCommon.cpp

@@ -12,6 +12,7 @@
 #include <Parsers/ASTSelectWithUnionQuery.h>
 #include <Parsers/ParserCreateQuery.h>
 #include <Parsers/parseQuery.h>
+#include <Storages/AlterCommands.h>
 #include <Storages/ColumnsDescription.h>
 #include <Storages/KeyDescription.h>
 #include <Storages/StorageDictionary.h>
@@ -42,6 +43,7 @@ namespace ErrorCodes
     extern const int NOT_IMPLEMENTED;
     extern const int LOGICAL_ERROR;
     extern const int CANNOT_GET_CREATE_TABLE_QUERY;
+    extern const int BAD_ARGUMENTS;
 }
 namespace
 {
@@ -324,6 +326,24 @@ void writeMetadataFile(std::shared_ptr<IDisk> db_disk, const String & file_path,
     out.reset();
 }
 
+void updateDatabaseCommentWithMetadataFile(DatabasePtr db, const AlterCommand & command)
+{
+    if (!command.comment)
+        throw Exception(ErrorCodes::BAD_ARGUMENTS, "Unable to obtain database comment from query");
+
+    String old_database_comment = db->getDatabaseComment();
+    db->setDatabaseComment(command.comment.value());
+
+    try
+    {
+        DatabaseCatalog::instance().updateMetadataFile(db);
+    }
+    catch (...)
+    {
+        db->setDatabaseComment(old_database_comment);
+        throw;
+    }
+}
 
 DatabaseWithOwnTablesBase::DatabaseWithOwnTablesBase(const String & name_, const String & logger, ContextPtr context_)
     : IDatabase(name_), WithContext(context_->getGlobalContext()), db_disk(context_->getDatabaseDisk()), log(getLogger(logger))

src/Databases/DatabasesCommon.h

@@ -23,6 +23,8 @@ void cleanupObjectDefinitionFromTemporaryFlags(ASTCreateQuery & query);
 String readMetadataFile(std::shared_ptr<IDisk> db_disk, const String & file_path);
 void writeMetadataFile(std::shared_ptr<IDisk> db_disk, const String & file_path, std::string_view content, bool fsync_metadata);
 
+void updateDatabaseCommentWithMetadataFile(DatabasePtr db, const AlterCommand & command);
+
 /// A base class for databases that manage their own list of tables.
 class DatabaseWithOwnTablesBase : public IDatabase, protected WithContext
 {

src/Databases/IDatabase.cpp

@@ -58,6 +58,11 @@ IDatabase::~IDatabase()
     CurrentMetrics::sub(CurrentMetrics::AttachedDatabase, 1);
 }
 
+void IDatabase::alterDatabaseComment(const AlterCommand & /*command*/)
+{
+    throw Exception(ErrorCodes::NOT_IMPLEMENTED, "{}: ALTER DATABASE COMMENT is not supported", getEngineName());
+}
+
 std::vector<std::pair<ASTPtr, StoragePtr>> IDatabase::getTablesForBackup(const FilterByNameFunction &, const ContextPtr &) const
 {
     /// Cannot backup any table because IDatabase doesn't own any tables.

src/Databases/IDatabase.h

@@ -21,12 +21,14 @@
 namespace DB
 {
 
+
 struct Settings;
 struct ConstraintsDescription;
 struct IndicesDescription;
 struct StorageInMemoryMetadata;
 struct StorageID;
 class ASTCreateQuery;
+struct AlterCommand;
 class AlterCommands;
 class SettingsChanges;
 using DictionariesWithID = std::vector<std::pair<String, UUID>>;
@@ -376,6 +378,9 @@ class IDatabase : public std::enable_shared_from_this<IDatabase>
         return database_name;
     }
 
+    // Alter comment of database.
+    virtual void alterDatabaseComment(const AlterCommand &);
+
     /// Get UUID of database.
     virtual UUID getUUID() const { return UUIDHelpers::Nil; }