Better code

src/Storages/IStorage.cpp

@@ -60,6 +60,8 @@ Block IStorage::getSampleBlockWithVirtuals() const
 {
     auto res = getSampleBlock();
 
+    /// Virtual columns must be appended after ordinary, because user can
+    /// override them.
     for (const auto & column : getVirtuals())
         res.insert({column.type->createColumn(), column.type, column.name});
 
@@ -81,13 +83,16 @@ Block IStorage::getSampleBlockForColumns(const Names & column_names) const
     Block res;
 
     std::unordered_map<String, DataTypePtr> columns_map;
-    for (const auto & column : getVirtuals())
-        columns_map.emplace(column.name, column.type);
 
     NamesAndTypesList all_columns = getColumns().getAll();
     for (const auto & elem : all_columns)
         columns_map.emplace(elem.name, elem.type);
 
+    /// Virtual columns must be appended after ordinary, because user can
+    /// override them.
+    for (const auto & column : getVirtuals())
+        columns_map.emplace(column.name, column.type);
+
     for (const auto & name : column_names)
     {
         auto it = columns_map.find(name);
@@ -422,8 +427,8 @@ void IStorage::renameInMemory(const StorageID & new_table_id)
 
 const NamesAndTypesList & IStorage::getVirtuals() const
 {
-    static const NamesAndTypesList VIRTUALS;
-    return VIRTUALS;
+    static const NamesAndTypesList virtuals;
+    return virtuals;
 }
 
 }

src/Storages/IStorage.h

@@ -172,6 +172,16 @@ public: /// thread-unsafe part. lockStructure must be acquired
     /// If |need_all| is set, then checks that all the columns of the table are in the block.
     void check(const Block & block, bool need_all = false) const;
 
+    /// Return list of virtual columns (like _part, _table, etc). In the vast
+    /// majority of cases virtual columns are static constant part of Storage
+    /// class and don't depend on Storage object. But sometimes we have fake
+    /// storages, like Merge, which works as proxy for other storages and it's
+    /// virtual columns must contain virtual columns from underlying table.
+    ///
+    /// User can create columns with the same name as virtual column. After that
+    /// virtual column will be overriden and inaccessible.
+    ///
+    /// By default return empty list of columns.
     virtual const NamesAndTypesList & getVirtuals() const;
 
 protected: /// still thread-unsafe part.

src/Storages/Kafka/StorageKafka.cpp

@@ -630,14 +630,14 @@ void registerStorageKafka(StorageFactory & factory)
 
 const NamesAndTypesList & StorageKafka::getVirtuals() const
 {
-    static const NamesAndTypesList VIRTUALS = {
+    static const NamesAndTypesList virtuals = {
             {"_topic", std::make_shared<DataTypeString>()},
             {"_key", std::make_shared<DataTypeString>()},
             {"_offset", std::make_shared<DataTypeUInt64>()},
             {"_partition", std::make_shared<DataTypeUInt64>()},
             {"_timestamp", std::make_shared<DataTypeNullable>(std::make_shared<DataTypeDateTime>())}
     };
-    return VIRTUALS;
+    return virtuals;
 }
 
 }

src/Storages/LiveView/StorageLiveView.cpp

@@ -618,10 +618,10 @@ BlockInputStreams StorageLiveView::watch(
 
 const NamesAndTypesList & StorageLiveView::getVirtuals() const
 {
-    static NamesAndTypesList VIRTUALS = {
+    static NamesAndTypesList virtuals = {
         NameAndTypePair("_version", std::make_shared<DataTypeUInt64>())
     };
-    return VIRTUALS;
+    return virtuals;
 }
 
 void registerStorageLiveView(StorageFactory & factory)

src/Storages/MergeTree/MergeTreeData.cpp

@@ -3610,12 +3610,12 @@ MergeTreeData::AlterConversions MergeTreeData::getAlterConversionsForPart(const
 
 const NamesAndTypesList & MergeTreeData::getVirtuals() const
 {
-    static const NamesAndTypesList VIRTUALS = {
+    static const NamesAndTypesList virtuals = {
         NameAndTypePair("_part", std::make_shared<DataTypeString>()),
         NameAndTypePair("_part_index", std::make_shared<DataTypeUInt64>()),
         NameAndTypePair("_partition_id", std::make_shared<DataTypeString>()),
         NameAndTypePair("_sample_factor", std::make_shared<DataTypeFloat64>()),
     };
-    return VIRTUALS;
+    return virtuals;
 }
 }

src/Storages/StorageDistributed.cpp

@@ -274,7 +274,7 @@ const NamesAndTypesList & StorageDistributed::getVirtuals() const
 {
     /// NOTE This is weird. Most of these virtual columns are part of MergeTree
     /// tables info. But Distributed is general-purpose engine.
-    static const NamesAndTypesList VIRTUALS =
+    static const NamesAndTypesList virtuals =
     {
             NameAndTypePair("_table", std::make_shared<DataTypeString>()),
             NameAndTypePair("_part", std::make_shared<DataTypeString>()),
@@ -283,7 +283,7 @@ const NamesAndTypesList & StorageDistributed::getVirtuals() const
             NameAndTypePair("_sample_factor", std::make_shared<DataTypeFloat64>()),
             NameAndTypePair("_shard_num", std::make_shared<DataTypeUInt32>()),
     };
-    return VIRTUALS;
+    return virtuals;
 }
 
 StorageDistributed::StorageDistributed(

src/Storages/StorageFile.cpp

@@ -602,11 +602,11 @@ void registerStorageFile(StorageFactory & factory)
 }
 const NamesAndTypesList & StorageFile::getVirtuals() const
 {
-    static const NamesAndTypesList VIRTUALS = {
+    static const NamesAndTypesList virtuals = {
         {"_path", std::make_shared<DataTypeString>()},
         {"_file", std::make_shared<DataTypeString>()}
     };
 
-    return VIRTUALS;
+    return virtuals;
 }
 }

src/Storages/StorageHDFS.cpp

@@ -340,12 +340,12 @@ void registerStorageHDFS(StorageFactory & factory)
 
 const NamesAndTypesList & StorageHDFS::getVirtuals() const
 {
-    static const NamesAndTypesList & VIRTUALS = {
+    static const NamesAndTypesList virtuals = {
         {"_path", std::make_shared<DataTypeString>()},
         {"_file", std::make_shared<DataTypeString>()}
     };
 
-    return VIRTUALS;
+    return virtuals;
 }
 }
 

src/Storages/StorageMerge.cpp

@@ -55,8 +55,12 @@ StorageMerge::StorageMerge(
     , source_database(source_database_)
     , table_name_regexp(table_name_regexp_)
     , global_context(context_)
+    , virtuals({{"_table", std::make_shared<DataTypeString>()}})
 {
     setColumns(columns_);
+    auto first_table = getFirstTable([](auto &&) { return true; });
+    if (first_table)
+        virtuals.insert(virtuals.end(), first_table->getVirtuals().begin(), first_table->getVirtuals().end());
 }
 
 template <typename F>
@@ -510,10 +514,6 @@ void registerStorageMerge(StorageFactory & factory)
 }
 const NamesAndTypesList & StorageMerge::getVirtuals() const
 {
-    static const NamesAndTypesList & VIRTUALS =
-    {
-        {"_table", std::make_shared<DataTypeString>()}
-    };
-    return VIRTUALS;
+    return virtuals;
 }
 }

src/Storages/StorageMerge.h

@@ -49,6 +49,7 @@ private:
     String source_database;
     OptimizedRegularExpression table_name_regexp;
     Context global_context;
+    NamesAndTypesList virtuals;
 
     using StorageWithLockAndName = std::tuple<StoragePtr, TableStructureReadLockHolder, String>;
     using StorageListWithLocks = std::list<StorageWithLockAndName>;

src/Storages/StorageS3.cpp

@@ -353,11 +353,11 @@ void registerStorageS3(StorageFactory & factory)
 
 const NamesAndTypesList & StorageS3::getVirtuals() const
 {
-    static const NamesAndTypesList & VIRTUALS = {
+    static const NamesAndTypesList virtuals = {
         {"_path", std::make_shared<DataTypeString>()},
         {"_file", std::make_shared<DataTypeString>()}
     };
-    return VIRTUALS;
+    return virtuals;
 }
 
 }

src/Storages/System/StorageSystemPartsBase.cpp

@@ -278,9 +278,9 @@ StorageSystemPartsBase::StorageSystemPartsBase(std::string name_, NamesAndTypesL
 
 const NamesAndTypesList & StorageSystemPartsBase::getVirtuals() const
 {
-    static NamesAndTypesList VIRTUALS = {
+    static const NamesAndTypesList virtuals = {
         NameAndTypePair("_state", std::make_shared<DataTypeString>())
     };
-    return VIRTUALS;
+    return virtuals;
 }
 }