Some intermediate solution

src/Interpreters/InterpreterAlterQuery.cpp

@@ -62,10 +62,6 @@ BlockIO InterpreterAlterQuery::execute()
             alter_commands.emplace_back(std::move(*alter_command));
         else if (auto partition_command = PartitionCommand::parse(command_ast))
         {
-            if (partition_command->type == PartitionCommand::DROP_DETACHED_PARTITION
-                && !context.getSettingsRef().allow_drop_detached)
-                throw DB::Exception("Cannot execute query: DROP DETACHED PART is disabled "
-                                    "(see allow_drop_detached setting)", ErrorCodes::SUPPORT_IS_DISABLED);
             partition_commands.emplace_back(std::move(*partition_command));
         }
         else if (auto mut_command = MutationCommand::parse(command_ast))
@@ -90,6 +86,7 @@ BlockIO InterpreterAlterQuery::execute()
 
     if (!partition_commands.empty())
     {
+        table->checkAlterPartitionIsPossible(partition_commands, metadata_snapshot, context.getSettingsRef());
         table->alterPartition(query_ptr, metadata_snapshot, partition_commands, context);
     }
 

src/Storages/AlterCommands.cpp

@@ -1003,6 +1003,11 @@ void AlterCommands::validate(const StorageInMemoryMetadata & metadata, const Con
         }
     }
 
+    if(all_columns.empty())
+    {
+        throw Exception{"Cannot DROP or CLEAR all columns", ErrorCodes::BAD_ARGUMENTS};
+    }
+
     validateColumnsDefaultsAndGetSampleBlock(default_expr_list, all_columns.getAll(), context);
 }
 

src/Storages/ColumnsDescription.h

@@ -121,6 +121,11 @@ public:
         return columns.size();
     }
 
+    bool empty() const
+    {
+        return columns.empty();
+    }
+
     /// Keep the sequence of columns and allow to lookup by name.
     using Container = boost::multi_index_container<
         ColumnDescription,

src/Storages/IStorage.cpp

@@ -102,9 +102,9 @@ void IStorage::checkAlterIsPossible(const AlterCommands & commands, const Settin
     }
 }
 
-void IStorage::checkAlterPartitionIsPossible(const PartitionCommands & /*commands*/, const Settings & /*settings*/) const
+void IStorage::checkAlterPartitionIsPossible(const PartitionCommands & /*commands*/, const StorageMetadataPtr & /*metadata_snapshot*/, const Settings & /*settings*/) const
 {
-    ///TODO alesap
+    throw Exception("Table engine " + getName() + " doesn't support partitioning", ErrorCodes::NOT_IMPLEMENTED);
 }
 
 StorageID IStorage::getStorageID() const

src/Storages/IStorage.h

@@ -361,7 +361,7 @@ public:
     }
 
     /// Checks that partition commands can be applied to storage.
-    virtual void checkAlterPartitionIsPossible(const PartitionCommands & commands, const Settings & settings) const;
+    virtual void checkAlterPartitionIsPossible(const PartitionCommands & commands, const StorageMetadataPtr & metadata_snapshot, const Settings & settings) const;
 
     /** Perform any background work. For example, combining parts in a MergeTree type table.
       * Returns whether any work has been done.

src/Storages/MergeTree/MergeTreeData.cpp

@@ -110,6 +110,7 @@ namespace ErrorCodes
     extern const int UNKNOWN_DISK;
     extern const int NOT_ENOUGH_SPACE;
     extern const int ALTER_OF_COLUMN_IS_FORBIDDEN;
+    extern const int SUPPORT_IS_DISABLED;
 }
 
 
@@ -1421,12 +1422,20 @@ void MergeTreeData::checkAlterIsPossible(const AlterCommands & commands, const S
     columns_in_keys.insert(columns_alter_type_metadata_only.begin(), columns_alter_type_metadata_only.end());
     columns_in_keys.insert(columns_alter_type_check_safe_for_partition.begin(), columns_alter_type_check_safe_for_partition.end());
 
+    NameSet dropped_columns;
+
     std::map<String, const IDataType *> old_types;
     for (const auto & column : old_metadata.getColumns().getAllPhysical())
         old_types.emplace(column.name, column.type.get());
 
     for (const AlterCommand & command : commands)
     {
+        /// Just validate partition expression
+        if (command.partition)
+        {
+            getPartitionIDFromQuery(command.partition, global_context);
+        }
+
         if (command.type == AlterCommand::MODIFY_ORDER_BY && !is_custom_partitioned)
         {
             throw Exception(
@@ -1456,6 +1465,7 @@ void MergeTreeData::checkAlterIsPossible(const AlterCommands & commands, const S
                     "Trying to ALTER DROP key " + backQuoteIfNeed(command.column_name) + " column which is a part of key expression",
                     ErrorCodes::ALTER_OF_COLUMN_IS_FORBIDDEN);
             }
+            dropped_columns.emplace(command.column_name);
         }
         else if (command.isModifyingData(getInMemoryMetadata()))
         {
@@ -1530,6 +1540,27 @@ void MergeTreeData::checkAlterIsPossible(const AlterCommands & commands, const S
                 checkStoragePolicy(global_context.getStoragePolicy(changed_setting.value.safeGet<String>()));
         }
     }
+
+    for (const auto & part : getDataPartsVector())
+    {
+        bool at_least_one_column_rest = false;
+        for (const auto & column : part->getColumns())
+        {
+            if (!dropped_columns.count(column.name))
+            {
+                at_least_one_column_rest = true;
+                break;
+            }
+        }
+        if (!at_least_one_column_rest)
+        {
+            std::string postfix = "";
+            if (dropped_columns.size() > 1)
+                postfix = "s";
+            throw Exception(ErrorCodes::BAD_ARGUMENTS,
+                "Cannot drop or clear column{} '{}', because all columns in part '{}' will be removed from disk. Empty parts are not allowed", postfix, boost::algorithm::join(dropped_columns, ", "), part->name);
+        }
+    }
 }
 
 MergeTreeDataPartType MergeTreeData::choosePartType(size_t bytes_uncompressed, size_t rows_count) const
@@ -2525,6 +2556,21 @@ void MergeTreeData::freezePartition(const ASTPtr & partition_ast, const StorageM
         context);
 }
 
+void MergeTreeData::checkAlterPartitionIsPossible(const PartitionCommands & commands, const StorageMetadataPtr & metadata_snapshot, const Settings & settings) const
+{
+    for (const auto & command : commands)
+    {
+        if (command.partition)
+            getPartitionIDFromQuery(command.partition, global_context);
+
+        if (command.type == PartitionCommand::DROP_DETACHED_PARTITION
+            && !settings.allow_drop_detached)
+            throw DB::Exception("Cannot execute query: DROP DETACHED PART is disabled "
+                                "(see allow_drop_detached setting)", ErrorCodes::SUPPORT_IS_DISABLED);
+
+    }
+}
+
 void MergeTreeData::checkPartitionCanBeDropped(const ASTPtr & partition)
 {
     const String partition_id = getPartitionIDFromQuery(partition, global_context);
@@ -2639,7 +2685,7 @@ void MergeTreeData::movePartitionToVolume(const ASTPtr & partition, const String
 }
 
 
-String MergeTreeData::getPartitionIDFromQuery(const ASTPtr & ast, const Context & context)
+String MergeTreeData::getPartitionIDFromQuery(const ASTPtr & ast, const Context & context) const
 {
     const auto & partition_ast = ast->as<ASTPartition &>();
 
@@ -3058,7 +3104,7 @@ MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVector() const
 }
 
 MergeTreeData::DataPartPtr MergeTreeData::getAnyPartInPartition(
-    const String & partition_id, DataPartsLock & /*data_parts_lock*/)
+    const String & partition_id, DataPartsLock & /*data_parts_lock*/) const
 {
     auto it = data_parts_by_state_and_info.lower_bound(DataPartStateAndPartitionID{DataPartState::Committed, partition_id});
 

src/Storages/MergeTree/MergeTreeData.h

@@ -24,6 +24,7 @@
 #include <Disks/StoragePolicy.h>
 #include <Interpreters/Aggregator.h>
 #include <Storages/extractKeyExpressionList.h>
+#include <Storages/PartitionCommands.h>
 
 #include <boost/multi_index_container.hpp>
 #include <boost/multi_index/ordered_index.hpp>
@@ -505,6 +506,8 @@ public:
     /// If something is wrong, throws an exception.
     void checkAlterIsPossible(const AlterCommands & commands, const Settings & settings) const override;
 
+    void checkAlterPartitionIsPossible(const PartitionCommands & commands, const StorageMetadataPtr & metadata_snapshot, const Settings & settings) const override;
+
     /// Change MergeTreeSettings
     void changeSettings(
         const ASTPtr & new_settings,
@@ -563,7 +566,7 @@ public:
     }
 
     /// For ATTACH/DETACH/DROP PARTITION.
-    String getPartitionIDFromQuery(const ASTPtr & ast, const Context & context);
+    String getPartitionIDFromQuery(const ASTPtr & ast, const Context & context) const;
 
     /// Extracts MergeTreeData of other *MergeTree* storage
     ///  and checks that their structure suitable for ALTER TABLE ATTACH PARTITION FROM
@@ -817,7 +820,7 @@ protected:
     void removePartContributionToColumnSizes(const DataPartPtr & part);
 
     /// If there is no part in the partition with ID `partition_id`, returns empty ptr. Should be called under the lock.
-    DataPartPtr getAnyPartInPartition(const String & partition_id, DataPartsLock & data_parts_lock);
+    DataPartPtr getAnyPartInPartition(const String & partition_id, DataPartsLock & data_parts_lock) const;
 
     /// Return parts in the Committed set that are covered by the new_part_info or the part that covers it.
     /// Will check that the new part doesn't already exist and that it doesn't intersect existing part.

tests/queries/0_stateless/01388_clear_all_columns.reference

@@ -0,0 +1,6 @@
+0	1	Hello
+0	1	Hello
+0	2	Hello
+0	2	Hello
+0	3	Hello
+0	3	Hello

tests/queries/0_stateless/01388_clear_all_columns.sql

@@ -1,5 +1,34 @@
 DROP TABLE IF EXISTS test;
 CREATE TABLE test (x UInt8) ENGINE = MergeTree ORDER BY tuple();
 INSERT INTO test (x) VALUES (1), (2), (3);
-ALTER TABLE test CLEAR COLUMN x;
+ALTER TABLE test CLEAR COLUMN x; --{serverError 36}
 DROP TABLE test;
+
+DROP TABLE IF EXISTS test;
+
+CREATE TABLE test (x UInt8, y UInt8) ENGINE = MergeTree ORDER BY tuple();
+INSERT INTO test (x, y) VALUES (1, 1), (2, 2), (3, 3);
+
+ALTER TABLE test CLEAR COLUMN x;
+
+ALTER TABLE test CLEAR COLUMN x IN PARTITION ''; --{serverError 248}
+ALTER TABLE test CLEAR COLUMN x IN PARTITION 'asdasd'; --{serverError 248}
+ALTER TABLE test CLEAR COLUMN x IN PARTITION '123'; --{serverError 248}
+
+ALTER TABLE test CLEAR COLUMN y; --{serverError 36}
+
+ALTER TABLE test ADD COLUMN z String DEFAULT 'Hello';
+
+-- y is only real column in table
+ALTER TABLE test CLEAR COLUMN y; --{serverError 36}
+ALTER TABLE test CLEAR COLUMN x;
+ALTER TABLE test CLEAR COLUMN z;
+
+INSERT INTO test (x, y, z) VALUES (1, 1, 'a'), (2, 2, 'b'), (3, 3, 'c');
+
+ALTER TABLE test CLEAR COLUMN z;
+ALTER TABLE test CLEAR COLUMN x;
+
+SELECT * FROM test ORDER BY y;
+
+DROP TABLE IF EXISTS test;